The finished card measures 5.5"x4". The outside of the card was self-decorated (like most of my papers some considerable time ago for the Design part of a 3 year City & Guilds Embroidery course that I didn't quite finish after 6 years...) being white cartridge paper with yellow & ultramarine mixed with black applied wet into wet - so the initial paint layer is more or less undiluted thick yellow just out of the tube with a tiny bit of water to make it more easy to handle, and whilst it's still damp, I've dragged a pretty thin & wet brush full of the blue over the top - which makes the paint spread out and change colour.
You have to watch out with black acrylic, true black is impossible, so the manufacturers made one with a red base and one with a blue base so you can end up with an unexpected result if you don't use the right one. I'll have used the blue based one here as I want blues/purples to come through.
I tend to layer up the colour on the self-decorated papers as it adds to the richness, they're even richer if you start off with a coloured background (like the cat stamping of the last few days). I tend to use an A3 (twice the size of A4) piece of paper as it allows 'sweeping' strokes and then you can pick bits you want to use rather than try & get it right on a small piece of paper for a specific project.
Inside the card, I've taken a piece of white cartridge paper and put a wash of diluted yellow acrylic on it and waited for it to dry. Then I've drawn alternate rough lines on it with a white candle (just a normal household one), and yellow and purple wax crayons (just kids ones). Then I've brushed a wash of very diluted yellow and ultramarine (separately with a thin household decorating brush) to cover any gaps. The wax acts as a resist and you get the original yellow paint coming through under the wax candle and the crayons colours coming through themselves. Then I think I blotted it with kitchen paper to dull it down - the alternative to leave as it is and you can get 'dew drop' marks from the paint that's been resisted by the wax/ crayons, depending on how thick your final paint 'wash' is. If you think you remember doing this sort of thing at pre-school, you'd be right....
I used an old yellow A4 file tab divider as the cardstock to hold it all together and the sentiment is stamped on the back in VersaColour indigo.