German does differentiate between "leicht" (weight) and "hell" (colour/brightness). They're not interchangeable.
Nein. The non-inflected form of "leichter" is "leicht".
I don't know what level you were at when you posed your question, but if you're at Level 16, you're likely to be far beyond this by now. Regardless, someone else out there reading this might have the same question. So, if that's the case, a refresher might be in order. This looks like a pretty good page for one on German comparatives:
Hope that helps.