I’m fairly sure that when a skill is red, it still has a normal value. It is just below the skill number it was at before.
IE if you have skill 7 (23%) and it rusts down to 6 (99%) it will display 7 (00%) until it reaches 5 (99%), at which point it will display 6 (00%). There is no difference in skill gain rate or experience expenditure- it simply fudges the display.