That's a pretty good score! (beats me at 12593)
The yellow and brown don't activly do anything, instead they show what will be red/green the next round.
As for they higher speeds, they do indeed increase, just slowly at first. (level 1 is 16 frames between them, 2 is 15, then 14, etc. after level 6 it waits two levels to increase speed.) By level 29 you move every frame. The level increase is based on every 5 rounds (returning to the center) because collecting more greens results in more points scored at lower speeds.
It probably would have been wise to include this info on the itch.io page. Thanks for the review.