This is due to the "Automatic text color correction" feature, introduced in v7.0.
You're still able to have "white on white": put white as the foreground- and
the background color of your text.
02/04/2010 - mIRC v7.0
86.Added check for foreground/background colors in control codes in text. If they are the same, the foreground color is changed to ensure the text is visible.
15/04/2010 - mIRC v7.01
2.Changed the foregound [sic] /background color correction feature so that it no longer applies if a ^K background color is specified.