Probably fits sets themed around 3- or 4-color factions.

Does this even need the assimilate keyword ability? Wouldn't it be more natural to have an assimilate term defined that means "a spell is assimilated to a color if you spend that color of mana to cast it"? The keyword ability doesn't contain any information that the two static abilities don't repeat.

You'll notice that several spells that technically get "assimilated" already exist (e. g. Ogre Savant), but none get permanent keyword abilities - you generally want a marker (e. g. a counter of some kind) as a memory aid.

Btw, generic mana comes first in any mana cost: .

The design is a little on the complex side with only limited depth.

If I'd wanted this kind of 3-/4-color play I'd immediately consider having a variant of it that checked for basic land types i. e. "As long as you control a Plains, ~ has lifelink." and I see no strong argument to favor this version over that.