When I first got GIMP it was to make custom desktop icons for my kids movies folders...I can't believe where it's gone from there! (I'm a 40 y/o carpenter that just decided to try and learn) I started out running through the tutorials on their homepage, and that was VERY informative and helpful. I ended up here
(after stalking as an unregistered user) for some answers to questions I just couldn't seem to crack, so you came to the right place! I don't really know what your end goal is on your image, or what your trying to do, and changing layer settings is a fairly advanced thing to be doing for basic re-coloring of a photo...your question made me think back to my first days, and this handy tut. This is right from the GIMP homepage under tutorials.
https://www.gimp.org/tutorials/Layer_Masks/ and
https://www.gimp.org/tutorials/Selective_Color/In these tuts they desaturate, and re-color certain areas. Always make a copy of the layer you're about to do something destructive to, so you can backtrack easily. I've also found that adding a layer on top and reducing opacity so I can see the lower layer, then upping opacity after coloring, works well. If your starting image has well defined lines, then you should prob be working with path tools (path to color, fuzzy select, paths tool or scissors select tool) to define a region, create a new layer, and utilize your path in the new layer. I've found that these tools are a way to bring your imagination to life, all it takes is learning how to operate the magic machine! Good luck, and I hope that was of some use to you.
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A day without learning something, is a day wasted.