hello :D
This is a really nice start!
Your palette: The darker shades are rather lacking in contrast. Your darkest purple and black bleed together, it would be most practical to take one out because the piece is not benefiting from having both. The current most saturated color may be a bit too bright. Highlights and shadows tend to be more desaturated, the mid color has most saturation.
The lineart: ? don't see any lines! The construction is fine for these simple shapes, if that's what you mean.
The Shading: The light Looks a bit strong and focused. Nothing wrong with that per se, but I assume you're aiming for the generic top left lighting study semi ambient mid strength light source Less of a focus on that individual top left point I'd say. On the rod thing, why the single pixel line on top? you're going for slightly reflective, I see, but even still, there is nothing waranted to put a highlight there. See, light on a flat plane generally produces a flat color, and occuring gradients are negligible in pixel art. the highlights go on the edges as seen here
Having reference is very important for learning lighting and stuff because you're trying to emulate real life, so you need to look at real life.
Spheres may actually not be the best thing to practice shading on with so many colors and not dither. Spheres are very gradient ridden, and anything more than blocked out two color shading and a highlight can get tricky because you have to position the blobs of color just right for it to read like a sphere and even then it doesn't look smooth.
