Howard - Font colour is assigned automatically to contrast with the colour chosen for each Category value in the stack. If you re-assign a lighter colour to the currently dark-coloured value(s), all the text will be black...

You can manage the colours assigned to all Category values in a given field by right-clicking on the value in a Sidebar filter, or from the central field/column management dialogue:

Data > Manage Fields > Options > Value order, colour...you will see a full list of the values in the field and can assign a sort order and custom colours to any value in the field.

Do not use the Field colour option just below, which applies to the field when used as a measure in the Bar View, and will have other uses in future builds.