Add tone colour option for Chinese
You could also use the tone number instead of a colour (1,2,3,4,0) as in yi1, jiu3
I suspect this can be implemented straight away, whereas adding colour might need a lot more work.
For beginner readers using tone colours is an excellent way of embedding this problematic issue students have with remembering the tones for individual characters into the reading process. Students will be able to acquire the correct tones more readily without even having to focus on this task. Could I suggest that if this is implemented that the reader be allowed to select the tone colour s/he prefers as different readers have different preferences.