If this is similar to what the reproduction lense company from Italy uses for some repro Alfa lenses, it will fade away in a few years (depending on how much you use the car after it's done) to near clear and will require re-application every time it happens. If your Spider (I am assuming that you are thinking about using it on the round side markers on the 1750 Roundtail) will be a garage queen when it's complete, I think these kind of paint would work just fine but if you are thinking about enjoying her outside of the garage often, it may not last long.
Several of us with Porsche 928's have been successful in changing the yellow turn signal light in our tail lights to red, using Testor model paint. It is an enamel, so doesn't damage the plastic lenses, can easily be removed with turpentine/paint thinner, etc. and the candy apple red or hot rod red comes VERY close to the red plastic in the brake light section of the brake/rear light combos. When complete, it gives a nice monochromatic appearance. I've had it in place for ~2 years with no change in color. 4-5 very light coats. Small spray cans available at most hobby stores.
West Chester, PA