cool sketchbook dude, love all these studies and stuff you've got goin on. That master study looks prity cool too, one thing i would mention while looking at your screen cap is to keep your master study and your study at the same size. Its very tough to replicate a piece if your working from two different sizes, plus you loose alot knowledge of composition, shape, anatomy etc etc from doing this. Master studies are tough enough without making it even harder for yourself
Maybe hammer some black and white photo studies too to practice those values. Your doing great though dude, keep at it!
In addition to what warburton said, start with a larger higher quality image as your reference. It seems from your screenshot that the reference image is pretty small, and it can be hard to see details that way.
Not to discourage you from trying complicated pieces, I think it might be more beneficial if you started with something simpler. I know for me personally that it was much less initimidating to start with something like a portrait or a simple landscape.
Hey man iI agree with the above comments, and okay just my two cents, I see that you have you're swatches open. Know that, no matter what the swatches will not give you the colours you need to do a proper master study, or really any study...you need to learn to mix your own. I mean I don't know if you are...I guess I'm just extrapolating from your screen shot. Eitherway...keep up the work man!
I won't be so soft. I have serious problems with your master study.
First of all, colors. You changed them dramatically. The original have soooooo lovely warm ones (well I can never know the original's colors but if the found reference looks good I'm pleased with it). The sky have a greenish tint, that's true but you draw it much more greener and less yellowish. The spear thing is in wrong position, it should be much more to the left. The figure has bad proportions, it's a normal slim guy in the reference. I'd use a much bigger brush, focus on the colors (I agree with Psychobuddy, why swatches? Mix them and compare. Colors are hihgly important. You easily do an artistically sketchy quick thing just to get only the colors right. I saw some well colored spots once and I could see it's a sunlit forest path ), details can wait.
The ground at the left border is slightly lower than the highest point on the camel's back. You painted it much higher. It's useful to compare such things because it's really easy to do it wrong. It's okay to change things sometimes but you need to know your stuff really well, I'd probably don't mess with a master painter's nicely balanced composition and whatnot I do that with obviously not great photos But maybe it's good if you don't focus too hard to reproduce the master's painting right, I tend to do that and then I feel I'm a copy machine
Oh my morning chaotic mind. Sorry, I hope I said something to think about too