When we choose a completely grey color (R=G=B) the text on the enabled button looks allright, but when the button is disabled, the text is almost white and therefore almost invisible!
See the image below, the text on the buttons corresponds to the expression used to determine the color.
The column of buttons to the left is attached to the checkbox 'Enable buttons 1', the column to the right is exactly the same, but bound to checkbox 'Enable buttons 2' to show the difference.
As you can see, when we change to RGB-values from grey (all RGB values equal) to one of the RGB-values that is different, the text remains visible!
Does this make sense?