Look at the image and here are the steps.
1. Open skin in sks (skinstudio)
2. Red arrow.
3. Blue arrow. Use the drop down arrow and change to "show me the skin defined system colors.
4. Green arrow. Disabled Text.
5. Purple arrow. This is where you can see it change in the preview. It also changes the text color of what you are asking. Note> Be careful as it changes a few places. So making it dead black or whatever may not work. Fine something in the middle that you can read. Basically just mess with the color to where you like it.
6. Text color change. you can see it's same as #4