I would colour the function names in, like you have done, but perhaps it would be a good idea to have 2 colours: 1 for user-defined functions and 1 for Excel's functions.

The case question is a bit of tough one. Initially I thought that you should leave the case unchanged, so people could tell when an Excel function is being used. But for functions like SIN, do you really need to know this? And what if you wanted to use capitals yourself, let us not go starting conventions that we're going to immediately break!

The problem with changing the case is that useful information can be lost - if camel case or acronyms are used - so case SHOULD be preserved...

With one exception... Excel functions which are only uppercase should become lowercase, like you are already doing... With one proviso - use 2 colours, so Excel functions can be distinguished.