You can add some animation to the label that tells which color to catch when the objective is being changed.
The pace is fine for my taste as it gives a nice balance to the challenge :)
Totally agree with the comment below regarding the idea :) you can also expand it to shapes, movement styles or switch the color/word focus as mentioned below as well :) really neat idea of challenging focusing abilities! I hope you will continue this game and we'll see how it progresses :)