Have read different colored anti-freeze shouldn't mixed, but, the local auto parts store has some which states it can be mixed with any color. However, can't comment one way or the other on the validity of the claim so distilled water might be the best choice as long as it doesn't drastically dilute the water-to-coolant concentration.
#NEVER GIVE UP
#AUDACITY OF HOPE
"Rather not run the risk of offending anyone, so let me just go ahead and offend someone". -PONY MAN
"When you write the story of your life, don't let anyone else hold the pen". -UNKNOWN