What does pants to be darkened mean?

CGI to darken the pants, obviously because they are to bright in the shot.

Remember this was a "pilot", which is a cheaply made version of the show so that network execs can see what the show would be like, if they green light it as a series. The key word is "cheaply", meaning cheap visual effects; no color correction, shading, erasing stunt wires, etc. There is another shot of nighttime traffic that says "insert police cars".

If the pilot gets picked up they'll go back and fix all the problems.

She wet them.
Or muddied her pants.