Changing sprite appearance based on its location on the screen

[max-reuter]

Suppose I’d like my agent to appear red when it’s on the left half of the screen, and as usual when it’s on the right half. When it’s partially in each region, I want it to appear red for exactly the part of it that’s on the left, and normal for the part of it that’s on the right (30/70, 1/99, 50/50, etc.). How might I begin to implement that?