The Meaning of Life, the Universe, and Everything.
Actually there are a lot of things you can't change in the game and you will come across more and more of them the deeper you get into resource packs. Just make sure to keep and eye on update logs so you know when you can finally do things that are currently impossible (ie jeb_ has stated one day resource packs will be able to model entities and not just blocks and items). Currently most color schemes can't be edited by packs like water colors, map colors, or dyeable armor colors. Right now I think foliage is the only thing with an alterable color map... maybe the sky, not sure if that's just optifine.
So if you already have optifine installed to minecraft, then continue on reading. If not, then you basically just download it as a forge mod or double click the .jar file to get it by itself. You can find guides online.
Now I don't really know if you can change the banner colors specifically, but it may work with the sheep or dye color. I'll explain later.
First, you take your resource pack, and (if you use 1.12.2 and below) add a folder called "mcpatcher" inside the "minecraft" folder, If you use 1.13, then you name it "optifine" It should look like this:
Inside the file, you can replace the color numbers, with the numbers you want the colors to be. You can use this to pick the color, and then get the number: https://htmlcolorcodes.com/. Take the #number, not any of the ones with letters beside it.
Try replacing the sheep colors and dye colors with you colors to see if it works.
Hopefully that wasn't confusing. You can look up guides online.
Thanks for the help! For now, i just want to complete my Lego resource pack, before adding extras. I only use official Lego colors, so i will change the color.proporties to match the banners and colored sheep.