I've wanted to get the longer smoke stacks for My DVD prime, but I didn't want it to look to weird with the silver and mirriored chrome on him. I made my own plus a few other small modifications.
The original 20th Anniversary DVD Optimus Prime to refresh our memories
First I painted over the out-of-place yellow on his head with some Tamaya silver.
I then cut pieces of 5.5 mm styrene, filed the inside edge on one side of both damn near paper thin, and finally spray them with Tamaya silver leaf. After the silver dried I sprayed them with regular clearcoat to dull some of the shine, and it almost perfectly color matched Prime. I then attached them with 2-ton epoxy and let the epoxy cure.
Last but not least, I painted his gun with Krylon Fusion. I used the Tamaya silver again to dry brush sections of his blaster.