I would think the paint wouldn't have good adhesion to the glass, will flake off when exposed to the repeatative heat cycling, would reduce light output and cause premature filament failure due to the paint trapping heat.
I went with the StealthAuto bulbs, NASA certified supplier does the vapor deposition coating (same process as the Space Shuttle bulbs), lifetime replacement for peeling/flaking and even bulb burnout. The brightest marker/signal bulbs I've ever seen to boot. $80 for all 6 bulbs shipped.

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'03 X5 4.4 Sport, last of the M62s (8-03 build date)
I believe in deadication to craftmanship in a world of mediocrity!