Mixing coolants isn't a good thing. Do you know specifically what type of coolant you currently have? If so then match it. If you don't know you could top off with distilled water if it's just a little low. Or consider this time for a flush and replacement. A HOAT coolant is recommended, you will find they are available in various colors though. Read the label to ensure it is HOAT, flush and refill. And keep the container or buy an extra so you know for next time what you have.