Best way to use test pots?

I put the same colour from the same test pot on two types of card - one the card provided by Resene, and the other white card from Warehouse Stationery - and they've come out completely different colours! The Resene one much paler and cooler, the other deeper and warmer. What's the best method for true colour without putting it on the actual wall? Argh!

And you have put two coats on each card?

Cover the card completely - any white edges will let you 'see' different tones, and definitely put on two or more coats - same as you plan to on the wall. And move them around, and also set then up in a corner to reflect into each other - the tone will deepen substantially doing that.

Thanks guys. Yes, I used two coats, but didn't paint to the edge. I think I've been staring at them so long my eyes have gone funny. Doesn't help that they're various shades of white!

It is the unpainted white borders all around the edges that help your eye focus on the reality of the double coated colour. If the card from Resene and from the Warehouse was exactly the same [card and whiteness] then two coats of the colour should look exactly the same. The unpainted border all around the edge of the testpot colour holds the colour away from any existing wall colour - when you pin it on the wall- and stops that colour from unduly influencing it and making you see if differently - and it does. Resene have a little brochure called Is this the Same Colour? It has two windows showing colour on the front. You will swear on a pile of Bibles that they are two different colours. You open the brochure - there is only one colour - close it- two different colours - what are you seeing? Two colours on the face of the brochure [1/2 yellow and 1/2 grey] influencing what you see through the windows.
I spend my life explaining this to people and I really love the 'light bulb' look in their eyes when they get it.