There seems to be quite a variety of different coloured coolants on the market today, even muddy brown coolant is available, so its important to ascertain if the colour has changed, or is normal.
Normally coolant should have a strong colour to it.
Mostly clear, with only a hint of colour.
Black coolant
Dave Bright, 25 Sep 2003.
The ratio for water based coolant should be between 30% coolant 70% water, or 50% coolant 50% water. The easiest mix is 50/50. Take a 5 Litre bottle of coolant and add half to the cooling system. Add the same amount of water to the cooling system, then top up the 5Litre bottle with water, and poor in as much as will go. What is left in the bottle can be stored, and used at any time, knowing that the cooling system will still have the 50/50 mixture.
This is an indication of insufficient coolant/water mixture.
This will result in a lower boiling point for the coolant, and is likely to suddenly boil over when the engine is under high load, or after a time of driving.
This colour is only likely to be seen on Diesel engines, and is a sign that exhaust gasses are entering the cooling system. This is due to the high levels of soot in the exhaust gases. A good check is to dip a finger into the coolant when its cold, and rub it on your hand. If it stains your hand and is difficult to wash off then it is soot.
Soot in the cooling system can only mean one thing. there is a minute crack in the Head, Block, or gasket. In the early stages this may be the only sign of a crack. There may not be the usual signs such as, oil/coolant crossover, or rough running engine. However it will cause the coolant to exit the expansion tank, and the MPG will drop significantly.