primars

is there like clear primars and white and gray?
if so which one is best?

i've never seen anyone use clear primer. however, as for white and gray primer, white should be used when spraying white or any other light paints. gray is generally used for anything darker. if by "which one is best?" you mean "white or gray?" it depends on your situation. if by best you mean brand, you can tell by this board that many are branded by tamiya. also, note that white primer is thinner than gray primer.

I believe Tamiya metal primer is clear.