Tankies are simply "people against colonialism" is a new take for me. Pfft.
But anyway, your question is an obviously false dichotomy. I can be against some subset of people who are against colonialism while still being against colonialism myself. Supporting shared beliefs while not supporting unshared beliefs is a logical position for anyone to take. "The enemy of my enemy is my friend" is a very unrealistic and asinine take in most real life situations.
For example, Libertarians and I are both ostensibly against war, the military industrial complex, and the US being a world police force. But Libertarians would also let this capitalist hellscape chew up and spit out as many people as the market demands in the name of profit. We are not friends.
Similarly, by your definition, tankies and I may both be "against colonialism" (and I'm just granting you that for the sake of argument, as I very much think that that is a bullshit take), but we do not agree on the means, the justification of violence, and the authoritarianism needed to oppose it. We are also not friends.