In just two lines, the speaker describes a world drained of color. The picture, which could be a photograph, a scene, or life in general, is stuck in black and white. It's flat and dull, and it seems it will stay that way "always." But then comes a small turn: "Until you come in it." The arrival of one person changes everything.
The poem never says outright that the "you" brings color, and that's what makes it work. We fill in the gap ourselves. The speaker seems to be saying that things only feel vivid and alive when this person is there. Without them, everything looks faded.
The mood is quietly tender, with a bit of loneliness underneath. The word "always" suggests the speaker has been waiting, or at least knows how colorless things feel without this person. The short length adds to the effect. Like the picture, the poem stays bare until the last line, when "you" finally walks in.