What ΔE means, and how close a close match really is

Every match on Swatcher now carries a small number beside its label: ΔE 0.8, ΔE 2.2, ΔE 4.0. It is the figure the label is made from, and it is worth knowing how to read it, because the difference between a 1 and a 4 is the difference between a tin you can swap and a tin you cannot.

What the number is

ΔE is pronounced "delta E" and it means difference in colour. It comes from colour science, not from paint, and it is the standard way of saying how far apart two colours are to a person looking at them.

A colour is stored as three numbers: how light it is, how far it leans from red to green, and how far it leans from blue to yellow. Put two colours in that space and the distance between them is the difference. The formula Swatcher uses is called CIEDE2000, which is the 2000 revision of that idea. Earlier versions measured the straight-line distance, which treated a small step in a blue the same as a small step in a grey, when your eye does not. The 2000 formula weights the distance by where the colours sit, so the number lines up with what you would actually notice.

The scale is roughly this. A difference of about 1 is the smallest most people can see at all, and only with the two colours touching. At 2 or 3 you can tell them apart side by side, and would still call them the same colour. By 5 or 6 they read as different paints. Past that the number just says how far off the nearest thing was.

Where the labels come from

The labels on Swatcher are cut from that scale, and they mean what they say.

• Under 1.5 is an excellent match. Painted on two walls of the same room, nobody would find the join.
• Under 3.5 is a good match. Side by side on a card you can see it, on a wall you would not think about it.
• Under 6 is a close alternative. The same idea of a colour, but a different paint, and you should be choosing it on its own merits rather than as a stand-in.
• Past 6 is not a match. It is still listed if it is the nearest a brand has, because "this brand has nothing close" is a useful answer, but it is a different colour.

Here is the scale on one colour. The closest Benjamin Moore paint to Farrow & Ball's Hague Blue is Brush Blue CW-675 at ΔE 0.8. Lick's Blue 07 is a 1.6. Fleetwood's Schoolhouse is a 4.0, and it is a lovely paint, but it is a lighter, greyer blue than Hague Blue and you would not mistake one for the other. Acres Hall's nearest is Graphite at 8.4, which is a grey. The strip at the top of this page is those five in that order, and you can see the drift.

What it does not tell you

The figure compares the colour each brand publishes for its paint. It is measured under standard daylight, on a flat sample, at the finish the brand measured. It does not know about your north-facing room, your matt against their eggshell, or the batch you were sold. Two tins of the same paint can differ a little from one batch to the next, so a ΔE under 1 between two brands is inside the range you already live with.

So the number is for shortlisting. It tells you which of thirteen brands is worth asking for a sample pot from, and which will waste the trip. The sample pot on the wall is still the last word, and always will be.

Why we show it at all

We could have stopped at the words. But "close alternative" covers a lot of ground, and the number is what lets you rank inside it: a 3.6 and a 5.9 carry the same label and are not the same offer. It is also the honest thing. If we are going to claim a Dulux paint is the same as a Farrow & Ball one, you should be able to see the measurement the claim rests on.