As stated in another replay whiptail uses the newt widget library which itself is based upon the s-lang library. The s-lang library gets the capabilities of the

As stated in another replay whiptail uses the newt widget library which itself is based upon the s-lang library. The s-lang library gets the capabilities of the terminal by querying the terminfo database using the terminal name. This is the same way ncurses or most terminal based libraries work. The terminfo database does contain info regarding whether a specific terminal supports color but doesn't contain info regarding whether there is an escape sequence for that particular terminal to determine if color is turned on. Instead separate terminfo entries for color vs mono are in the database. Setting TERM to xterm when you are using xterm-mono is the error here. It fails just as setting your TERM to xterm when connected to a ADM-3A would cause things to fail.