I used those square buttons for the view buttons on my first driving cab. Also used the rectangular version for coin buttons.
The top part of the button is clear and snaps onto the base with a removable flat insert in between.
Easy peasy to stick a piece of paper in there or apply a decal to the insert.
The insert being removable also makes it easy to trim the label to size.
For the square exit button on my nephew's cab, I masked off an X then spray painted.
Knowing what I know today, I'd use Avery 18665 full sheet clear labels for everything.
Only thing of note that I can think of is that the colored rectangular ones I bought sometimes had white inserts and sometimes had color matched inserts.
EDIT: These were done using the avery labels. Note the use of color on the xbox button. The rest were left clear except for the letter.
