I have a Volkswagen Jetta 2008 S, and I recently noticed that the coolant level is near minimum. My user guide told me to get the purple G12 coolant and that was what the auto parts store made me buy. However, the one I currently have in my car looks more like orange or pink. Does the color change when it's diluted or should I be concerned about mixing different coolants?
I am not familiar with G12 coolant, as it seems to be VW/Audi specific, but mixing incompatible coolants will cause precipitation ("sludge") within your cooling system, which will cause blockage. G12 coolant comes in a variety of colours, read more here.
What you can do is remove a small quantity of your coolant (use a turkey baster), place it in a clean empty clear plastic bottle (disposable drinking water bottle) and add the same amount of your purple stuff. Shake it vigourously for about 25 seconds, then let it sit for 3 minutes then look at it. If you are seeing just one uniform liquid with nothing floating at the top or sinking at the bottom, your fluids are compatible. If you are seeing 2 different layers, large patches floating or big globs at the bottom, your fluids are not compatible.
If your fluids are not compatible or if you are not sure, get your cooling system completely flushed. The tank, the radiator, the engine block and the heater core. Then fill it with water and drain the entire thing again. Repeat until the water comes out clear and no bubbles. After your last draining, fill your cooling system with the coolant you prefer.
G12 coolant is indeed a VW/Audi-specific coolant, and pink/red when in the jug. The coolant is concentrate and needs to be mixed with 50% distilled water before filling the cooling system.
Usually we're not perfect when flushing and refilling the cooling system, and forget about the water in the heater core lines, or in the water pump. As a result the coolant becomes more diluted than the 50% ratio and the color fades to pink.
If your coolant is orange, then I would consider flushing the whole system and refilling; check to see how close you are to the lifetime/mileage of the coolant from your last serving. In my 2000 VW Passat, the G12 coolant turned orange as it aged and reacted/collected deposits from the engine and cooling system. You will often see sediment at the bottom of your coolant reservoir resulting from this. For a DIY, check your local Jetta user forums for the model of your car. The procedure generally involves draining the system, refilling and circulating with distilled water, draining again, then refilling with coolant (once drained water is clear and free of sediment).
On a side note, VW/Audi now only sells G13 coolant, which is the non-toxic/bio-degradable upgrade to G12. It is purple in color, and 100% backward-compatible and mixable with G12, and also has a longer lifespan. I purchased a gallon container from the local dealer for about $20 USD.