So idea is to let you continue for some time and not be intrusive and not change anything (OK, anything important at all). Suggestions like chaning fonts or graying the screen are just not that.
Besides color, what else could be done ? Here is some brainstorming from me:
1. THe more you work, the slower are keys rendered, just as when computer slows down. This allows for some continuation but eventually you will stop to let computer (and yourself) rest.
2. The more your work, the more random, very short, beep sound is inserted on key presses. The same benefits as above unless you turn off the volume in which case the good old BIOS beep could do. This may deconcentrate a bit tho.
3. The more you type the more mouse starts moving on its own. Eventually it just flies around like crazy (until you use it, then when you don't it starts again). Yes you can still continue working like nothing happens.
4. The windows border colors start changing, the more you work, the more frequent it gets. It can be less distracting then mouse flying around.
All of above wont change a thing in your workflow which is important here as you want to allow some continuation and you don't actually want to change anything for user (screen size, font size, brightness changes, etc. are all intrusive and can change word ordering, syntax coloring etc. something you may depend a lot upon ( I do ).