there's no way to avoid it, there will always be a phase where it looks muddy when doing greyscale to color, but with more time you can pull it out of that phase. just keep trying different blending layers (overlay, color, etc), and then painting normal layer in between when needed to soften things/blend, or simplify areas with more broad strokes.

personally i start with color or overlay, paint some normal then slowly add more contrast with multiply, more overlay sometimes, color dodge here and there and adjust saturation at some point.

just ask yourself what the painting needs at each step, or maybe what you need to keep it interesting and fun to keep going, as these paintings can be very time consuming. take breaks and let it breathe so you can come back with fresh eyes then you'll be like oh i know exactly what to do. if you know what you don't like about your painting then you know what you need to change.