When the toolbar is overpopulated, you have to click the little expansion arrow to make the rest of text formatting toolbar slots visible.
In this popup-toolbar, there is no little black arrow which shows you the color-picker for the font colors.
Normally, while the font color slot is on the main toolbar instead of the popup toolbar, you can press the little black arrow to show the color picker.
It would be nice to also access the font color picker when the font color slot is opened in a popup toolbar.
*** This bug has been marked as a duplicate of bug 42029 ***
In the latest master a solution was implemented. You may test it with the nightly build.