Blue or Red Franking Ink? Which Colour Your Machine Needs

If you have ever stood in front of a franking machine holding a cartridge and wondered whether it should be blue or red, you are not alone. It is one of the most common questions we get, and getting it wrong can mean your mail is surcharged, delayed or returned.

The short answer: blue for Mailmark, red for older meters

Royal Mail approved blue ink is required for all Mailmark and Smartmeter franking machines. Royal Mail approved red ink is used by older, pre-Mailmark digital meters that still print a traditional analogue impression. The colour is set by the type of machine you have, not by preference, and you cannot substitute one for the other. If your machine prints a 2D barcode block alongside the postal indicia, it is a Mailmark machine and it needs blue.

Almost every franking machine sold in the UK today is Mailmark, which is why the vast majority of the franking machine supplies we ship are blue cartridges.

Why Royal Mail cares what colour your ink is

It is tempting to assume the colour is cosmetic. It is not. Royal Mail sorts hundreds of millions of franked items through automated Mail Centre equipment, and that equipment has to read your impression at speed, on the first pass, from a moving envelope.

Approved ink is tested, not just coloured

Royal Mail tests franking inks through its Mail Centre Automation before approving them, and requires that ink is bought only from authorised suppliers or their retailers. The approval is about how the ink behaves once it hits paper: how sharply it holds an edge, how quickly it dries, how it survives being rubbed against other envelopes in a mail bag, and how it reflects under the scanners. An off-the-shelf inkjet refill can be exactly the right shade of blue and still fail every one of those tests.

Blue simply reads better

When Royal Mail moved franking to Mailmark, blue was chosen because it showed better readability in testing. That matters most for the 2D datamatrix barcode, which carries the item data Royal Mail uses to verify and track your post. Royal Mail's Mailmark specification asks for a print quality of ISO 15415 grade 4/A or 3/B, with a barcode module size between 0.5mm and 0.7mm. A tiny, high-contrast barcode leaves no room for a washed-out or bleeding impression. If you want the full picture of how the system works, our guide to what Mailmark means for UK franking users walks through it in plain English.

How to work out which colour your machine takes

There are three quick checks, and any one of them will usually settle it.

1. Look at an impression you have already printed

Frank a blank envelope and look at what comes out. A Mailmark impression includes a square 2D barcode to the left of the postage details and the Royal Mail cruciform. An older analogue impression has no barcode block and typically shows a town circle or wavy line pattern. Barcode means blue.

2. Check the cartridge currently in the machine

The safest single check is the part number printed on the cartridge you are replacing. Manufacturers issue separate part numbers for red and blue versions of what is otherwise the same physical cartridge, so matching the number carries the colour with it automatically. If the label has worn off, the model name on the front of the machine is enough for us to identify the right consumable.

3. Check the age and status of the machine

Royal Mail has been retiring non-Mailmark franking for years, and machines that cannot produce a Mailmark impression are increasingly rare in active mailrooms. If you are unsure whether your meter is still supported, your maintainer can confirm it. Royal Mail publishes a directory of authorised franking companies who are permitted to inspect, maintain and repair franking equipment.

What happens if you use the wrong colour

Two things tend to go wrong, and neither is quick to fix.

The first is that Royal Mail's automation cannot read the impression. Unreadable items get pulled out of the automated stream and handled manually, which can lead to your mail being delayed, surcharged, or returned to you as unpaid. You have already spent the postage, so this is money out of the door twice.

The second is mechanical. Franking machines use a specific ink formulation matched to their printhead. Putting a non-approved ink through the head, or filling a cartridge with something it was never designed for, can clog the nozzles and damage the head permanently. That is a repair bill, not a consumable cost. Our article on original versus compatible franking ink explains where the real quality differences sit, and why a properly manufactured compatible cartridge is a different proposition from a generic refill.

Buying the right cartridge first time

The quickest route is to shop by machine manufacturer, because each brand's cartridges are grouped together with the correct colour already assigned:

If you would rather start from what other mailrooms buy most, our best selling franking ink cartridges cover the models we ship in the highest volumes. And if you are still not certain, send us your machine model and we will confirm the correct cartridge and colour before you order.

Getting the best out of the colour you are using

The right colour is only half the job. A correct blue cartridge can still produce an impression Royal Mail struggles with if the rest of the setup is working against it.

Envelope stock matters more than most people expect. Heavily textured, very dark or highly recycled paper absorbs ink unevenly and softens the barcode edges, which is why approved meter posting envelopes exist. For anything that will not feed cleanly, such as a padded mailer or a parcel, use franking labels rather than trying to force the item through the machine.

Storage matters too. Cartridges left uncapped, stored somewhere cold or kept past their date will underperform whatever colour they are, as we cover in our guide to how long franking ink lasts. And if your impressions have started coming out patchy, that is usually a printhead issue rather than an ink colour issue, which our walkthrough on fixing faint or streaky franking deals with step by step.

One practical note worth knowing: Royal Mail allows franked mail to be posted up to seven days after the date printed on the impression, so a batch you franked on Friday is still valid the following week. Full guidance is on Royal Mail's franked mail preparation page.

Frequently asked questions

Does Mailmark require blue ink?

Yes. All Mailmark and Smartmeter franking machines must use Royal Mail approved blue ink. Blue was selected because it produced better readability in Royal Mail testing, which matters for scanning the 2D Mailmark barcode.

Can I put red ink in a Mailmark franking machine?

No. Red ink is not approved for Mailmark impressions and Royal Mail's automated equipment may not read the barcode, which can lead to your mail being delayed, surcharged or returned. Red approved ink is only for older, non-Mailmark digital meters.

How do I know if my franking machine is Mailmark?

Print a test impression and look for a square 2D datamatrix barcode next to the postage details. If the barcode is there, the machine is Mailmark and needs blue ink. If there is no barcode, it is an older analogue meter that uses red.

Can I refill a franking ink cartridge myself?

Refilling with non-approved ink risks clogging or permanently damaging the printhead, and the resulting impression may not meet Royal Mail's readability requirements. Use a genuine or properly manufactured compatible cartridge made for your machine instead.

Is compatible franking ink allowed by Royal Mail?

Yes, provided the ink is approved and supplied by an authorised supplier or one of their retailers. Royal Mail tests approved inks through its Mail Centre Automation so that franked mail can be recognised and sorted reliably.

Not sure which cartridge your machine takes? Browse our full range of franking machine ink and labels by manufacturer, or get in touch with your machine model and we will point you at the right one.

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