I am running v9 on mac pro and think that I am doing something wrong.
When commenting the solution of a particular exercise I add a text cell (command 7) and type the comment which is nothing more than a sentence with some mathematical expressions (control ( - expression- control )).
To change the size and color of the whole comment, I simply highlight it and Format - Size or Color - to the desired size and color. The change does not apply to whole comment! Why? (I think that is due to the expressions).
For instance, if the comment is as follows
Here is an example, if $P = 1$ then the whole solution depends on the number of ...
the change applies up to $P = 1$. The rest of the sentence is unchanged.