Hi, welcome to Pixelation.
Are the colours and shade good enough for my creation?
That will always be up to you. I like to ask myself if the lighting makes sense. And when considering a particular area, a good question is: what is this trying to portray?
So for example, those highlights on the bottom edge. That lip is on the underside of the helmet, in the back, and Vader wears a black suit which doesn't reflect much light—in other words, it's physically impossible for light to be present there, in that manner.

What a tricky subject... there are so many ways this helmet can be lit. Here is my advice, which can generalize to all pixel-specific problems. You have a limited number of elements with which to represent something; this demands sacrifices. The best solution in most cases is to simplify as much as possible, allowing for clarity, and only then add a level of detail which does not compete with the subject as a whole.
A related idea is the tradeoff between detail and noise, for example, highlights. If you have high contrast bits peppered across lower contrast areas, it confuses any visual priority. When creating a highlight or any single-pixel texture, you should consider if its benefits outweighs any noise it creates.
My first thought was that you make good clusters for being so new to this, most people are quite timid about that for a few years. Keep it up. Also, value is what really matters in correctly rendering subjects; so colorblindness can potentially be used to your advantage.