To be exact, Im referring to the fact Terramorphic Expanse and Ash Barrens use a colorless land frame while Prismatic Vista and Fabled Passage use the multicolor land frame despite all four being basic fetches (side note: I want make a pumpkin spice joke here, but those are actually pretty tasty).

Is it because of the rarity? If so, do you feel that future designs of any rarity should use the same frame?

The explanation that I've heard in the past is that the gold frame goes to lands that can give you any color of mana this turn, which is true for fetchlands that can give you an untapped land. I'd guess Ash Barrens gets ruled out because it's permanently colorless once it's on the battlefield.

Ah neat; thanks Cmdr Jim