If you found a Colossal planet (not gas/plasma) with decent stats would you consider it worth a planet flux?

Generally... yes.

Depends on the stats and whether you feel the planet can be defended as is. I would agree and say yes in general.

yes 100% as you can always upgrade planet to being a 20x arty in feature with terra's and crystals if a 17x col. plasma goes for over 2k ctp then even a plasma is worth a planet flux depending on who the buyer is

Is an 18x Mining Colossal worth it?

What's its artifact production? If it's poor, might as well start a colossal plasma as a project and save the flux.

Hmmm... okay. Thnx!