I accepted a peace treaty where the AI gave me a planet, and i want to assimilate the colonists on that planet to my race. But despite building the alien management center and being dictatorship government, the colonists wont assimilate at all regardless of how many turns i wait. Whats going on?

Must be MoO2.

If you are playing v1.4xx for DOS (GOGs version) it should be so that planets given to you like that are instantly assimilated. Only conquered planets need assimilating. Colonists that are not assimilated yet have a different stance (as if hands in their pockets).

Im playing MOO2 1.50 but the planet didnt instantly assimilate to my race....

Can you explain a bit more?
Do you mean that the pictures of the people are of yellow colour?
Or are they pictures of another race and they don't turn into pictures of your race?
What is the effects in terms of production and farming? is it lower than expected, i.e. you get a penalty?

/ if you have a save game of it, i can take a look at whats going on.

Oh, sorry I did not try out the new fan patch yet, the 1.50beta. Just finished all my ongoing games in MoO1 so MoO2 will be next up when I feel like mooooing ;-), then I will try it.

Hopefully someone can help you :-)

Ah nevermind i think i misunderstood. I thought assimilation would allow you to change the enemy race to your own (So for example sakkra to meklar).

It is good that the game doesn`t .... just move some guys from a subterran race to all planets where colonists don't have that trait ;)

No, in MoO2 (and MoO3) it is normal to have multi-race empires.