@Lup0z: It's a shame no one answered you several months ago when you posted your apple. Muddiness generally comes from strokes being too transparent and overlapping too much. When the individual parts lack definition, that's when things get muddy. You'll notice that in bumskee's example, he starts off with very opaque strokes. Each area is dominated by a single color. After that, you zoom in and carefully blend the edges between the strokes.
@P0int12lank: Looks like a good start. It's lacking sharpness in some areas.
@Villainbynecessity: Yours is a good start as well. The main things I see to work on here is contrast and shadow. I'd recommend doing some in grayscale to get a better handle on it.
@Lagaard: As you said, looks kind of flat. It's mainly a lack of contrast and having very little difference in brightness between areas in light and ones in shadow. Also with the shadow on the apple, I'd pay closer attention to the shape the shadow makes. The edge will typically only be sharp near where it starts, then get blurrier the further away from the object.
@quockhanhlk: It looks like a solid start. You've got a good amount of contrast between the light areas and shadow. The main thing I'm noticing here is the surface is all one hue. I'd recommend getting a closer look at the surface of the real one and working on reproducing it's pattern in the painting. Some more blending practice couldn't hurt either.
Spent around 6ish hours tonight on a still life of a potato. It found it pretty challenging and wasn't really sure how to handle all the little bumps on the surface: