colored plastic

I have occasionally used Testors dullcoat or glosscoat over small parts to avoid painting them if the color of the plastic was right. It gives a sheen that eliminates the translucent plastic look.  Never done that for large pieces, though.

Yes it's best to prim with a near but different color, in order to see how your color coat is even.

Nice nic model.

If the plastic is a similar colour to the final finish, in this case orange, I'd suggest a white overall primer coat first for two reasons.

1: To cover any dissimilar colours caused by fillers etc.,

leading to

2: To ensure consistent application of the final colour. Painting orange over orange plastic, it may be difficult to see any areas where your top coat may have missed or been applied too thinly allowing the underlying plastic colour to show through.

I’ve never heard of anyone using clearcoat as primer. Why do you want to do that? I would use regular primer instead of clearcoat. You can get primer in white, grey and black, even pink.

My go to primer is Rustoleum primer in rattlecan.