Why do people suggest painting testers on paper, they look completely different

[T3nessee]

I painted testers on the wall prior to plastering . Then thought I’d check the one I picked with the new kitchen and test on paper so as not to ruin the new plaster and it looks completely different. Decorator is coming this week!😫

How many coats are you doing on the card? I do 3 coats and find it’s pretty accurate

I've always found card samples to be 100% accurate. But I always use plain white A4 card (not bits of brown cardboard box) that's thick enough to withstand a few coats of paint. I use 2 or 3 coats on my test sheet (so I can be confident that the card will not show through - I've haven't yet found a paint that will give really good coverage with just one coat), and I also paint the card right to the edges. That way, you don't have any of the original card colour showing - as PPs have noted, it can make the paint look different in contrast.

The other advantage of using good quality homemade A4 paint chips like this is that you can file them all in plastic wallets in a ring binder, and then you have your own personal library and don't keep buying testers of the same colours years later!

To be honest, there's no reason at all why paint samples would look different in plaster or on card unless it was interference from the base colour. There's nothing magic about the substrate! If you've painted your samples properly (2-3 coats, taking care to get a good finish with each one), then you won't get any show through and they'll match.

we painted testers on to pieces of plasterboard (roughly 50cm by 30cm) - did them double sided to maximise usage. They were brilliant in helping decide which colours to use.