I wouldn't hesitate to paint/draw a few spots on a plain t-shirt - I get fed up with them dictating what type of mufti they should wear. I can see that it generates a bit of excitement for the kids but it's a PITA and if you have to buy something specially it negates the point of giving money to charity. Get your DC to join in the painting so you've got his buy-in and aren't just presenting him with something he might turn his nose up at.
Or if you are that unsure about painting, put some sticky dots on the T-shirt, our post office sells about 200 of them for 99p. It's only got to last a couple of hours.