Orange on black must have primer underneath but like many times before this was explained, primer is not only for better look of colour but also to prevent the paint from pealing of the body once it is cured (so like I wrote before in one thread primer is there to give the paint something to hang on). So take any kind of acrylic primer for Tamiya acrylic (just don't take that dupli colour transparent) if it is gray primer you should put a coat of white paint before orange to have a nice looking orange.
There is a catch of course Tamiya paints on Tamiya bodies work well even without primer but if the body is molded in dark colour and you want to apply the light colour you will still need a primer.
But this is all in FAQ and I will get flamed because writeing it down again
