Some good news! I just found out why this is happening.
It turns out that the selected video (the one that’s turning into the unusual colors) was located below a track that was set to “overlay.” Because of this, it affected all the videos below that overlaying track. I figured this because I inserted a new video track on the very top, and all the colors were completely normal–until I moved it past and below the overlaying track, and the colors changed.
I removed the overlay on that track and now everything’s normal again. Thanks for the help, Joel! However, I would like to know if there’s some way to set a track to overlay and prevent that undesired effect from happening.