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 dozens of lenses with 49mm filter size for various mounts, most often for Pentax or Sony. Also, some compact cameras use that filter size, e.g. Fuji X100 (with either AR-X100 adapter or an extra 49mm filter frame).
Also, there are two other possibilities not yet adressed here. Ignoring the debated usefulness or lack of of color filters on digital systems, this is well discussed in the linked question.
49mm filters can be used with a step-up ring on modern lenses that have a 40.5mm, 43mm, or 46mm thread, IN SOME CASES (eg full frame lenses used on APS-C) a step-down from 52mm or 55mm could be usable too.
Adapted manual lenses (arguably modern, arguably not modern :) ) are probably even more common now than in 2011, and 49mm was a very common thread size on SLR lenses.