I would recommend adding one extra color at the time, eventually using the “trois au crayons” technique of the old masters.
If you want to stay close to pencil I would start by adding a middle tone with color pencil to the “black pencil” you’re already drawing. You then have white (paper)-middletone-black (pencil)
One option is to draw with pencil and white chalk on toned paper
Next step would be to add a warm red to add the complexity of color temperature relations.
You then have white-middle-red-black – and can get images feeling very full of color using this very limited palette. Once you’re comfortable with that, you could take the dive into color theory.
Use the same principle for oil painting, one color at the time. But also remember to have fun and “go wild” for the fun of it from time to time. Best of luck 🙂