Gels fade after a while.
I work with theatre, not film, but generally, after a 2 week run I chuck away all the old gels. That's about 10 x 2 hours + technical rehearsal time. (but not all lamps are on at the same time). Having said that, if we're setting up at a new venue, and there's a lamp with the right gel already in the right place, hey, leave it in, I won't bother cutting a new one.
Theatre gels are also much more strongly coloured, hence they fade faster, and also the lights are sometimes more high wattage - one production, I was putting 13 000 watts on stage, and maybe 20-40 % of that was absorbed by the gels - (darker gels absorb more).
Film uses more naturalistic gels for colour balancing.
If you're just starting out, one of the main things that will help is a heat resistant gauze - drape it over your film lights (aka diy work lights) when indoors - it will diffuse the glare. Big pieces of cardboard will help you to mask where you don't want the light to go. It's useful to have a friend(s) to stand around holding that cardboard - works better then fiddling with a stand.
Try taping white paper over one side of the cardboard, then you have a choice of a shiny side or matt side. You can then use that reflection as a secondary light source if you want (aka fill light).
One other semi-pro trick I remember from my limited film work is if you're doing any filming indoors, and there's a window with daylight in your composition, try taping a neutral density gel over the whole window - helps balance intensities and gives you better definition on what the camera sees of the outside environment.
There are books out there that will help, but some of them are over-technical. The main thing for you at your stage is to work with what you have, make the most of it, and try new things, learn how things work. Try not to obsess with spending an hour trying to get a small lighting detail right - if it doesn't work, move on.
Hope that help.
