they're state laws covering that. so it's best to look them up for your state.
however, while searching for info on my state (va) before i bought and installed them on my dd'er, va requires them to project amber light. so i can use bulbs that aren't amber but still emit amber light. the trouble with amber bulbs is that when daylight hits them, the reflective backing of the clear housing reflects the sunlight going though the bulb causing what people call the egg yoke effect.
they make some expensive silver bulbs but i found a writeup on a BMW forum on how a guy made a DIY version of the silver bulbs.
here's what i ended up with:
the writeup i found did it a little different. he was masking off 3 triangles on the sides of the bulbs and laying down heavy coats of enamel down. then when he removed the tape, he would have amber triangles on the side. i'm not sure how well they work in the shape of a BMW's turn signal housings, but because ours are more flat, the light output flat out sucked. so i chose to do a very light coat of enamel. this diffuses the sunlight shining through them, but doesn't block as much of the amber light when they're in use.
here's a link to my full writeup: