Who quoted you £500 plus vat ? Was it a big body shop ? Take it to a couple of back street bodyshops and get a few quotes , you'll be amazed how much the prices differ .
Imo if they are saying its because its a special silver paint thats why it costs so much they are talking b****cks , thats if its standard factory Renault paint .
Are you doing the whole bumpers or just the insert . If its just the inserts , wing mirrors and side strips i'd do it myself . Get the paint made up at a local motor factors in rattle cans .
Wash the bits down your going to paint with fairy liquid . That'll remove any grease / dirt .
Then rub them down with some 320 grit wet n dry .Wipe down with a damp cloth then dry or use a tack cloth .
Then a couple of coats of plactic primer .
Then rub that down with some 1000 grit wet n dry .Wipe down with a damp cloth and dry or a tack cloth .
Then 3 ,4 , 5 coats of top coat . Depends how thick you put it on . Try not to put too much on at once to avoid any runs . Leave it to dry for a couple of hours .You shouldn't need to rub down the top coat , unless you have any runs .
Then 2 or 3 coats of lacquer . If you put the lacquer on too thin it will come out a slightly matt finish , too thick and it will run and can become slightly cloudy . If you have a shed it would be best , with a heater in ( not your bedroom , lol ) or you'll choke everybody to death in the house and have a fine dusting of paint all over your bedroom .
Leave the lacquer to dry for 48 hours . If the finish isn't perfect you can rub it down slightly with some 2000 grit wet n dry and then polish up with some meguiars scratch X .
N.B. When you rub down , always clean it will a slightly damp cloth or tack cloth to remove any dust then dry off . Also a drop of fairy liquid in the water when you use the wet n dry helps to lubricate it .