An oddity...red v. yellow render lines

So here's an oddity.

I have 16fps AVI footage from film transfers that I'm importing. (BTW they're the MJPEG codec AVIs that were crashing in an earlier build. They work fine now. Thank you.)

I create a new sequence from clip, which comes in as 4:3, 2048x1536, 16fps. I convert the sequence to 24fps HD 1920x1080. I set the clip to scale to frame size, then apply Optical Flow time interpolation to smooth out the frame interpolation cadence. The render bar turns red. If I then scale the picture up a bit, the render bar turns yellow.

I find that curious. Wonder why?