Cel shading is a great start but yeah, it is very simple color. Nothing wrong with that, there's a lot of people out there who have others do their base colors for them. it's a good foundation but there's nothing wrong with expanding your abilities either
Whenever I feel like I need to spice up colors i usually go looking at screenshots from movies, i'm particularly fond of the colors in Atlantis and The Iron Giant but any movie/show/game, animated or not, that has colors you like can be a good place to draw inspiration from! Also there's tons of tumblr blogs that have color pallets on them you can try out to challenge yourself. :0
I always go to animated movies first because its usually very obvious what the characters' base colors are, and then in other scenes it helps give you an idea of how different kinds of light interact with color and change the base colors, darker colors blend together in dark blue lighting and lighter colors can blend together in bright settings. YE. RAMBLES. I love colors but idk if any of that was really helpful lol
Try starting out with a specific color in mind rather than using the Usual Base Colors n stuff! Think about where your character is and how the background's colors will effect the character's colors.
If you're using a warm color for the lighting, the shadows would be a cool color. Tho u should feel free to experiment with that cause I think the way you use certain colors can change the "mood" of the drawing aaH
BASICALLY even if you're not planning on shading a drawing, just giving it an overlay or multiply layer in a certain color can add a lot of atmosphere to it

this was a lot haha hope u get the answers you're looking for tho! ;o;

Thanks for this! I definitely will have to look at animated movie stills to get a bit of a better idea on things