Hey there,
I'm on Version 3.7.7 of NGUI Pro, and I believe to remember that using bbcode, I was able to output any color of text regardless of base Component tint.
Right now I observe a behaviour that just tints the base tint of the rendered text with the desired color, instead of replacing it. An example: I want to have a tint of the label of #ca9b6a, and in the second line of the label, I want a text in white.
I tried it as shown in the following picture, which had no effect:

And instead of replacing the lower part with grey, it just darkens the brown color of the label here:

It works fine if I invert the colors, meaning I use a white tint and just colorize the brown parts:

As I said, this might also be my memory fooling me, but I believe that was different in previous versions. IMHO it is rather impractical to be forced to have white tint for any type of multicolored label text. Therefore I would vote for this behaviour to be changed to a more intuitive "replace" instead (or fixed if it is unintended). Or at least make an option available to switch between the two behaviours of tint and replace.
I just wanted to add that NGUI is one of the greatest extensions to Unity I ever encountered, documentation could be better, but functionality is simply top of the notch, super flexible and yet super powerful, thank you very much for creating this!