Bug Reports: On editing page
The A and ab for Font and Colour, both offer colour options!
The 2 colour options you're speaking about cover 2 different aspects of the text. The A button selects the colour for the text itself and the ab button selects the colour for the background of the text.
If you edit a page and try both buttons you can see what effect they have within the WYSIWIG editor but then don't save your changes so you don't affect your live site.
Cambridge Open Systems' Support Team