It's a process that sounds complicated but is actually super easy to learn and exploit; gradient maps take a specific value (black to white and all the greys in the middle) and assign it a color. By adjusting where those lil ^ arrows sit on the gradient, you change which value is assigned that color, thus changing what part of your drawing is that color.

all the gradient maps I used are community made presets I downloaded for free using the CSP community asset portal

now pair that with layer masks, and you can isolate where on the picture the gradient map is in effect. Below is the gradient map that's Just the red for the couch, a few character accents, and the tail;

above in the orange is the layer mask button as well as where it is displayed on your layer (in CSP) once you apply it. When you want to change your layer mask, you click that second square on your layer; with that selected, you now can control what part of that layer is being shown without actually changing whats on that layer.

With the layer mask selected, you can erase whats there, then go over it with a brush or a selection tool and add it back as if it was never erased at all. The positive parts (what is visible on the picture) are displayed in white, the negative parts (what's been erased) are displayed in black. Obviously you Have to make sure you're interacting with the layer mask and not the layer itself (the layer itself is selected in the screen cap above, that's why it has that in-program white outline) or you really will just erase shit you can only get back by hitting 'undo'.

then you can add more gradient maps above or below the first one, depending on what looks best;

this is the bottom most gradient map layer arranged in my folder of gradient maps, because I wanted this orange as the foundation of the pic, the rest building off it.

the last gradient map layer is smushed between the red and the orange, and this is how I got those blue shadows and tail accents.

there is a Single value assigned that blue, but the gradient aspect allows it to be more versatile, and the browns making up the majority of the gradient help it feel like an extension of the orange layer. It's important to make sure your gradients harmonize like that otherwise it becomes more obvious they are two separate gradients interacting rather than the illusion of a more complex, selective color palette. While you can get a lot of depth out of a single gradient map, when you layer them, you're able to easily add shading, quickly change the color of a specific part of the picture and overall just cheat your way to a very cohesive, intricately colored picture with WAY less effort and color picking on your part.

taking this;

and turning it into this;

with 3 layers 8'>