Interestingly, until I added the button text, a message box titled
"Microsoft Visual Basic" appeared every time I clicked on the button, with
just 400 on it! There was an OK button and a Help button, but the Help
button didn't do anything when I clicked it. But when I added the text
(and after I deleted the text), this no longer appeared.

Just twigged why (very slow!). Of course, the code changes text colour, and
if there's no text... But it's an unusual VB error message, as it doesn't
halt the code and go into debug mode. That's what confused me.