if its brown it might be a good time to give the rad a flush while you are at it, drain the old stuff by taking out the plug under the rad here :
Disconnect top rad hose and put your garden hose into it + run until the water coming out is clear. Refit rad hose + plug, fill rad + overflow bottle to the mark with coolant. Let the car idle up to normal temp then cool completely to "burp" it, then top up if needed.
If you want to completely flush the system, stick the garden hose in the bottom rad hose which goes into the block instead.
Steve