I have found four color-filters in my desk: intense red, orange, light green and light blue. Are there lenses with 49mm filter size which these could be used with?
Usage of color filters in digital photography has already been covered in another question.
There certainly are at least 21 lenses with 49mm filter size, all of them for Pentax and Sony. Also, some compact cameras use that filter size, e.g. Fuji X100 (with either AR-X100 adapter or an extra 49mm filter frame).