"but it might have been too difficult due to the limited amount of colours"

 The point of dithering is if you have a limited pallete and want to create a brand new shade, tricking the eye. The dithering here is over-done and gives the pony a snakeskin like texture. Notice how the right mountain looks better than the left mountain. The shapes are more defined and clear. The strength of this pallete IMO comes from the thickness of the colors, like the rainbow or the trees that you did. Those look fine.

Your armor designs remind me of the suit in Vanquish. Good stuff.