The point of value is to reveal the form of the shape, its volume and depth. You'll have lots of trouble trying to do that with that line art. The hair looks flat. An easy way to help yourself trying to color is to draw with considerations of the front, side, back, and top/bottom of something (hair and face in this case) in mind when doing the line work. Then revealing that with value is much easier and not superficial. Alphes colouring does have lots of that, colouring somewhere just to give texture and interest, but his lineart, the base, looks good and allows for those elaborations. And I think it looks great.
I'd imagine he coordinates his colors with filters/blend modes to unify the palette, but I don't know what they are. So in the mean time once you get the lineart cooperating, you could simply use the palette from some existing alphes style picture to see what the artist was doing. Then perhaps screw around with layers and blend modes to see if you can recreate a palette from just the base colors. Picking the colours themselves shouldn't be too hard, it's the colouring what where. That takes taste and practice, and isn't exactly something someone can easily point you towards, aside from the considerations of form, shape, and texture as mentioned above.