@Neal_Blackburn You have really creative robot designs there, very different and interesting. I also like how you change the viewer's angle and perspective.
I will write what I think about the digital part but please keep in mind that it is just my opinion/taste.
I agree with Asya that the local colors are very saturated. That itsself is just a choice but you run into a problem with the shadow colors. Shadow colors are in most cases a bit more saturated than the light colors/midtones. You basically seem to add black for shadows which is not as appealing as colorful shadows. And shadows in reality do have color. I have two approaches, either go darker and more saturated with the local color or create a multiply layer for the whole image, chose one shadow color (light and not too saturated) and draw the shadows in on teh multiply layer. There are more ways but these are the basic ones I think.
The second thing I see is that you use the soft brush a lot which is very typical for artists starting out with digital drawings. The general advice is to use this brush only when you really need it. Hard shadow edges can be very appealing, a mix of hard and soft shadow edges is best.
Try out all the brushes and decide which appeal you want to get. You can go for lots of texture with a traditional media look, you can go for an inked/comic style with only hard shadow edges and flat colors, you can go for a realistic look or of course any mix. But I would try to achieve different styles so you understand which aesthetics are possible.
And have fun! Digital drawing has endless options and it is so much fun to explore these 