I too have successfully used these stencils on the seats of several bikes. The paint I used with success was the Krylon Fusion brand for plastic. On a DOHC 750, it matched the colour exactly to a stock 1974 CB350F that I have by using the "42338 Nickel Shimmer" colour that Krylon comes in. This shade as on my CB350F (and likely similar to other SOHC-4 models) is somewhat silvery, almost a bronze shade, rather than white as I seem to recall the earlier Hondas from the 60's. At any rate, what I used on the DOHC 750 looks identical to the 350F.
