WHen spraying a plastic bumper, can I get away with just using plastic
primer all over it then basecoating it and lacquering it ? (tis metallic)

Or do I have to put another regular primer on it ?

Its a moondust silver colour for basecoat.

Ahh yes the plastic primer I have is grey. I have some high build primer
   which is yellowy orange I think and the top coat is a silvery
metallic colour. Now can put the basecoat on a yellow primer and not
have a colour shade problem ? or would I need some grey primer on the
high build or would I need to buy some high build in grey ? :).....my
neighbour has confused me a bit as he said you have to put the correct
colour of primer to get a proper finish.

Neighbour who claims to know the ins and outs of spraying recons, just
"slap" some topcoat on the plastic primer and it'll look fine.

Advice appreciated.