So why is the color change "permanent"?

[bplus]

Values passed to a sub retain any changes made in that sub.

Which I thought shouldn't happen unless the variables were explicitly shared or were returned as part of a function call. Darn it really has been ages since I used QB.

That is new to QB64, arguments in a Sub or Function were passed by what they call ByValue in older versions of QB but QB64 does them ByRef. So if you are going to change a variable value it is best to copy the incoming value into a variable for the Sub to work with and leave the incoming variable alone after.