Options of Button Labeling
I've always found the modern-day label makers to be very useful for this for prototypes. There are a variety of color options by changing relatively inexpensive tapes.  You can also make front panels with the text cut right into it, if you want to spend the time and money, like at frontpanelexpress.com.  

Of course, you can buy switches with whatever label you want on them, if you're buying large quantities, but I imagine that's not cost effective for prototyping.

You can buy switches with programmable displays that use LCDs or OLEDs!

As an aside, good for you concerning yourself with usability issues!  Every engineer should read The Design of Everyday Things and think about their human interfaces!