I think the value in the higher tier worlds are the resources such as plentiful iron, gems, gold, silver, titanium, thorns, corruption, and future items such as blink and darkmatter. Also the hunting for higher level meteors and the associated larger loot drops. That seems like plenty to keep the interest for people to go there. I do not think having more color varieties on a tier 1 or tier 2 would have a significant affect on traffic to the higher tier worlds.
I agree the developers have done a good job making the planets not look the same with the biomes, tree sizes, waters and canals and such. I will disagree that they have very different colors of rock. There are too many repeats of exactly the same color and too many times they are just shades of a different color versus coming from a different part of the color wheel.