I see the visibility rules in the form editor only let you set plain text, not a rich text label.
I'd recommend putting something like this in the page's CSS
.mktoLabel {
color: white;
}
or, if that doesn't work, try:
.mktoLabel {
color: white !important;
}
If you're using a custom color that isn't exactly white, replace "white" with whatever the color's hex code is (i.e. #ffffff)