The code for this assignment is available here.

None of the spheres are emissive, and the background acts as an infinitely large emissive sphere. The line where the yellow and magenta spheres meet looks red because that is the only color reflected by both spheres (same for the green and blue lines). The center looks black because each sphere has one color that it doesn't reflect. When a ray hits all 3 spheres, the cumulative reflectance will be (0,0,0).

This image is very similar to this image from the last assignment, but it is a bit more washed out because rays that hit the dark areas have a chance of bouncing around a bit to find a path to a light source.