You do not need to change system font colors to change the button text color.
1. Go to the button menu and click on the text tab.
2. There is a dropdown box for the different states the button has. You can change the font color for each state.
3. This is where you can change the color of the font.

It is basicaly the same for the font color in the menus.

The reason you would skin a per pixel frame is if you wanted it to have shadow or transparency. I have not done that yet, so can't advise you on that. The UIS2 frames are regular frames that dont have shadow and transparency. Thats what I skinned on my first blind.
Hope this helps a bit.