Your drawings are so cute! Especially the first one +_+
Here are some pretty basic rules when it comes to coloring/shading, if any of it doesnt make sense im sorry LOL
-Keep in mind a direct light source~ It will cause less confusion and more unity/give a more concrete picture if your light is from one single point instead of shading in multiple and different directions. Sometimes people forget where their light source is so they draw lines to remind them (like in this left example)
-Dont be afraid to use darker colors!
-The bigger the difference in contrast between the base color and the color youre shading with, it implies a stronger and more intense light source. (example would be being indoors with sunlight shining through window vs standing directly under the intense sunlight during summer)
-If you'd like to go for a less 2d/flat look, pull your colors closer to the center of the object, it will give it more shape/dimension.
-Also, this is in japanese but in regards for choosing what color to shade with, it's basically saying
instead of just using a darker more black version of, for example, blue, it looks more vibrant if you moved to a slightly violet shade! And then for the bottom example, when using yellows shade with a more orangey color (sometimes people even use purple!) Changing the hue when shading can make a HUGE difference.
Hope this makes sense!