Hi,
So I am playing for Terran. I got several Lurkers. I have sufficient colonization technology. When I am trying to send them to colonize free planet through the Galaxy context menu > Planets tab I got the following message:
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It's a free planet, but you can't colonize it right now. You have sufficiently developed Colonization technology, but you require at least one Colony ship. Distance to planet 1414
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But when I open the Fleet page and select Lurker, it is possible to send her for colonization of the same planet:

I believe, it must be possible to use Galaxy context menu to send a ship of different race for any mission it is capable to perform.