goodday,
I am trying to paint red crosses on my academy ambulance, but I always end up with pink paint mixture. My method goes like this:
1) paint the panels red
2) take some masking tape and cover a red cross
3) paint the 4 remaining squares white
but when I do this, the square turns pink

the first time this happened, I left it alone and let it dry for a night and day. I tried to paint it again but it turned out again pink. Is this because I should leave it drying even longer or could it be something else.
I use tamiya red enemal thinned to paint it better and as white I use humbrol enemal.

thanks for any help

Few ideas...
Reverse the order, painting red over white
If the paints are blending, apply a clear barrier between coats
Use less thinner in the white
Apply several light coats of white
Use decal sheet for the cross, painting the white square then cutting out a cross of red decal paper
Use different types of paint, carylic over enamel of vice versa.
Are you hand or airbrushing?

thanks for your suggestions. I think I'll first try using different mediums.

I paint them by hand

I can't imagine the frustration of trying to get coverage with white paint applied by brush. If you don't have an AB, at least go get a rattle can of white (plain old flat white from the hardware store will do), mask your cross, give it a couple of light sprays and get on with your life.