My first thought is French, because the sheer influence France had on the world in this period and the extent of their bureaucratic system, which I suspect would result in there being tons of primary sources in French.
I also feel like Spanish would useful, but perhaps not as useful as French.
German would probably help too, but because of their limited influence outside Europe I feel like German wouldn’t be too useful for broadening perspectives beyond Europe.
I suppose it depends on when/where you are really imagining when you say ‘colonial period.’
Ive obviously found great use for French, but for instance if you’re thinking about the early modern period then Spanish or Portuguese would be great languages to learn.