The title does a lot of work here. To be colonized is to have something taken over from outside, and the poem suggests this happens not only to land but to the mind. The speaker says "we" pay "in good oil" for visions that "they" sell back to us. The oil hints at resources being drained away, while the visions seem to be ideas, images or ways of seeing that arrive packaged by someone else.
Then the poem turns inward. "I now know / how blind I am" sounds like a sudden, uncomfortable realization, not a boast of wisdom. The final lines shrink the scale: the speaker's thoughts are "small" and they "labor / against the night." That image feels tired but stubborn, like a tiny lamp pushing at a huge dark.
The mood is quiet, bleak and a little defiant. The speaker admits powerlessness, yet the fact that thinking continues at all gives the ending a faint, fragile persistence.