In my experience, Tamiya really holds well, so if you are just changing color on a new, very clean model, I wouldn't worry about a primer (as long as it isn't glossy).

That said, if you changed things or sanded or something, it couldn't hurt to get a solid coat on there to make the top coat even.

Also if you ran it or it's noticeably dirty, a good wash couldn't hurt. I use Simple Green added to hot water a tub and dunk whole sprues before building anything. It's a perfect solvent for mold release, skin oil, etc. and after a good rinse I've never had trouble with any paint adhesion, using primer or not.

I find when I open these HL tanks, they are bathed in oil on the gears, recoil, airsoft, etc so it takes some care to keep the surface clean before painting if you are not going to disassemble.