@james will correct me here (he created the palletes) but I will attempt to explain and give a little more information on how we choose world colours. We have 255 colours to choose from. We then have different algorithms to create different palleted from these 255 colours. Each algorithm is designed with some goal in mind. For example we have algorithm that try to encapsulate: earth like pallettes, monochromatic palletes, low saturation palletes etc. Worlds are given palletes based on their level and world type. This is why level 1-2 worlds looks similar. The idea here is that when you travel to a new world, they look very different from a chromatic perspective. We wanted to achieve the feeling of discovering something new. This means that for each given world type and for each given block there is a limited set of colours which it can take. Now, we are continually trying to improve palletes (as with all areas of the game). The worlds we added most recently have 'improved" pallettes. The colour space for each planet and block should have been increased quite a lot, reducing the chance of duplication substantially. That is not to say you will suddenly see neon pink starter worlds.
There is more we want to do with the pallete system and I’m sure we will continue to iterate. As far as white rocks go, I’m not sure this is something our current palletes can achieve. Maybe they can I have no idea. I’m pretty sure at some point there will be white rock however it is very possible that they will only be found on planets of certain tiers and types.
Additionally, we plan to add some form of dying system by which you can extract the colour of a block and apply it to another. This doesn’t help you now but will improve the colour situation in future.