Textures are considered a part of the detailing process - something you do last on your piece.
Since you have your lineart (which is pretty nice, btw!) I recommend that you colour it with a single, midtone colour (if he's not going to be all brown, then one color of each hue in case of differently colored parts) and then block out the general shading - it doesn't have to be precise yet, it's purpose is to show, what geometry, what 3d shapes is your character made out from.
Since it's a stone golem, you might want to go with blocky/cube shapes.
Excuse the roughness, but it's just supposed to give you the idea
Of course not perfect cubes, but when working with rocks, think of them as blocks first.
The texture so far, reads in my mind as more of a pillowy surface - like it's edges drop down smoothly. Rocks most of the time have a rough texture or a layered appearance. Another picture to illustrate:
As far as the colors go: I like the muddied-reddish base color, for lighter colors you could warm it a bit to an more orange hue for example. Observe how the lights and shadows shift the color in real life.
Oh, and the current three shades are very close together value wise - don't be afraid of contrast - it's something that makes up the readability of sprite and makes them pop
If the difference between too colors is too much, you can always add an in-between color. If you're just starting with pixelart, I wouldn't worry too much about the number of colors, but try not to get too overboard
In general, you should focus on making things look good first - using lower color counts and clever colors/palettes comes with the experience, don't worry about it yet.
Hope this helps, good luck!