I am finding myself having the same problem as the original poster.
The color solid, or color, or even a plain white picture displays when I play my timeline, but when I export the video, it doesn’t show.
Basically, I changed the crop on the color solid (it looks like a white bar) and I move it across the screen using keyframing. Like I said, it shows fully rendered, but not in the final output. I have even tried different colors!
Any suggestions?