If you are referring to the coolant that is in the overflow bottle, it could be normal buildup of crud.
Remove the rad cap and inspect the coolant inside the rad. If it is the same then you may have trouble.
If not, just empty and clean the overflow bottle then fill with fresh coolant. When was the last time he flushed? Should do every three years even with extended orange type. If he uses normal green coolant, the chocolate milk is corrosion product....rust from the cast iron block. Change and flush it.