The most expensive - not the best)) but I have the copiсs, grays.
The main advice is to draw more often. And try to mix the markers. See what colors are obtained by mixing blue and yellow. Pink and blue? And to do gradients from one color to another. As soon as it is clear how the markers work, drawing should no longer be a problem.
I looked at your drawings - you have talent, but you do not have enough basic knowledge. If you want, I can try to help.