forum.datacad.com

If your Mtext is all a single color, you can use the normal Change, Color to change the contents to a single color as with other text entities. But if you have an MText entity which you have taken the time to use the handy multi color settings of Mtext within it, Datacad protects those settings (which can often be labor intensive) and will not allow you to globally change that individual Mtext's colors. If you do need to change an entire multicolored MText, you can still open the MText editor and quickly Ctrl+A to select all of the text and apply a new color. MText is smart and safe!

Just did a quick test.
I thought maybe color mask would help, but it doesn't.
Also, if you have changed a few words to different colors, say
red, white, green, red, blue... with red being the default layer color, if you change only the 1st red will change, not any additional times that color may occur.
Would be nice if mask would penetrate into MText.