You have some really good looking studies here.
It is not at all unusual for one's grayscale work to be better than stuff in color. In grayscale, you only have to worry about the relative values of light and dark. It is simple, and that is why artists have traditionally started out using just charcoal or single color chalk until they have mastered the basic chiaroscuro techniques.
When you introduce color, the difficulties increase exponentially. All of a sudden you have to worry about choosing the correct hue, how colors blend, warm and cool contrasts, local color variations which have identical values, and the list goes on and on...
These things take time to learn so don't get discouraged. Spend alot of time painting from life and really focus on color matching. If you are working digitally, copy photos and try to pick the correct color by yourself, then check your guess against the color picker tool. That might give you some insight. Most of all, just keep painting!
As the ego shrinks, so the spirit expands.