Hi there, you have some cool stuff going on here :) As others have pointed out, it seems you are struggling with edges a bit, which is understandable as they can be hard to get right.
In addition to what others have said, perhaps try the following exercise: When doing a study, first paint only hard shadow edges, no blending. When you feel like you have captured the main shadow areas (Don't be afraid to simplify the light/shadow masses at this stage), selectively blend the edges that need to be blended. Check the reference for this and really try to differentiate between what is a hard edge and what is a soft edge. Think about the underlying form: More curved surfaces will have softer edges where light goes to shadow, (More blending over a larger area) surfaces with hard contours will have harder edges (Less blending), so consider that and for each edge, think about why it is the way it is.
Right now I think you often try to blend almost all shadows in a small area (Going from light color to shadow color very quickly), which makes the color transitions look muddy.
Don't get discouraged though, keep practicing the way you do and it'll come along nicely, it's cool to see you're doing so many master studies :) Keep drawing and posting!