Mixing different brands of coolant is pretty much a guarantee of trouble. There are several different anticorrosion packages out there which react badly to one another's presence.

Old-style green Prestone uses silicates, which are doubly bad: one, they form gummy crud with nonsilicate coolants, and two, the seals in Honda water pumps aren't silicate resistant and will be eaten away by them.

I recommend that you use only one type of coolant, best bet being Honda OEM coolant, second best being a nonsilicate retail coolant, and use distilled or purified water to cut it. (Unlike tap water, distilled has no minerals to precipitate out and clog passages -- also, distilled has no corrosive chlorine or fluorine or both).

The owner's manual usually spec's distilled, although even dealers usually ignore it and use tap. It's cheaper for them. And hey, if the cooling system blows because of tap water reactants, from the dealer's perspective, either you may give their service department $$$$ to fix it, or give the front end sales group $$$$$$$$$ for a new car. Either way, they're happy.

If there's a mix in a system, I recommend repeated flushes with tap water from a hose, trying to force crap through and out of the system. Get everything out. Don't flush a hot motor or add cold water to a hot motor!!! The block or head may crack.

Chemical flushes would be a last, last, final resort. They can attack stuff like, um, head gaskets -- as a friend found out.

Once your flush is running clear tap water, then cycle in distilled until almost everything in the motor is pure distilled. Fill, run, drain, wait. Fill, run, drain, wait. Remember to run the motor in between flushes to open the thermostat. Remember to have the heat full on so that the stuff in the heater core circulates and gets flushed, too.

When you have only clean, clear distilled coming out at the end of all this (a full day, usually, given cool-down time between cycles), it's time to add coolant.

Not all of the water will come out when you drain, so you can't just add coolant without doing some balancing math. Here's the simplest hack around. Assuming you want 50/50 coolant/water, do this: look up the total system fluid capacity. If it's, say, 4.5 liters, add 2.25 liters of uncut full strength coolant, then add distilled slowly until the system is full up.

Don't dump used coolant in the gutter. It contaminates wetlands, and pets like to drink it -- it'll kill their kidneys, then kill them. Either take it to a hazmat point, or at minimum pour it down a toilet to a sanitary sewer. The bugs in the tanks at the sewage plant apparently do OK eating glycols.