Hey, thought I'd have a go too (I'm new as well

), it seemed like a good exercise as shading is one of my weak areas.
original | edit


in steps

I thought in the original the boundaries of the color changes didn't match the form of the reference, so that was the main focus of my edit.
The shadow also seemed wrong (wrong angle and shape) and although you got the blue tint in the topside shadow, you missed the green tint on the bottom.
I still don't think I got the shadow right, but its a step in the right direction (I hope

)
Also, I know I dithered the hell out of it, but I felt it was necessary as the reference is very porous.