If you know what radiator fluid should look like, you can decide when maintenance work on your vehicle is necessary. Driving with poor or inadequate coolant can overheat your engine and reduce its operating life. However, you cannot determine the condition of the coolant solely by visual inspection.
Radiator fluid is a clear liquid; however manufacturers add dyes to differentiate the various formulas. The most common colors are orange, green, red, and yellow. Do not mix different color fluids, since they are designed for different conditions and may react with each other and ruin your engine. Even fluids of the same color from different manufacturers may not be compatible, so it is important to be consistent with the brand you use.
Coolant manufacturers add corrosion inhibitors, such as silicates or phosphates, which raise the pH of the coolant solution. However, these inhibitors gradually degrade over time. As coolant ages it becomes less transparent and may contain visible deposits of metal dust or bacterial sludge. You must test your own coolant using a litmus strip to determine how much reserve alkalinity remains. If you determine that your fluid has expired you should flush your radiator as soon as possible.