Ages ago I used to have some translucent red "varnish" for painting inside the existing lens to give it more colour which I am sure came from Halfords - my local branch is denying all knowlege and I can't remember what it was called - but like samuria, I have a use for it if anyone can come up with a name/source please.
If you use the Granville paint try a small drop on a non visible part of the lens. The paint is intended for glass not plastic and it may melt or damage your lens.
Humbrol to the rescue - remember those little tiny tins of paint you used to paint your plastic kits with? Colour no 1321 is "Clear Red" and no 1322 is "Clear Amber". If you google "Humbrol paint colours" it brings up a load of options from about £1.50 a tin BUT the postage is more than that. If you've got a local model shop I'm sure they would order them for you. A 14ml tin should do two lenses I would have thought. If you get stuck, pm me and I'll see if our local shop will order it.
I've been down the shops and bought a tin of red and amber clear Humbrol paint (see codes above). The red is fine straight out of the tin - I tried it on a sheet of glass and the light shines through it. The amber was a different matter and I had to thin it c25% with cellulose thinners before it remained opaque - used straght from the tin it was opaque at first but not when it dried. Be warned that it + the celly thinner that I used to clean the bristle brush that I was using caused the bristles to go soggy and stick together - suggest that you get some cheap kids paintbrushes from whatever your local replacement for Woolworths is and anticipate binning them after one use! I've only tried them on glass so try an inconspicuous area on any plastics first. Oh, and if you are my age, you will probably need a magnifying glass to read the instructions on the label.....
Bert (pseudonym).