Drain it, fill it with straight water, run it for a few mins, dump it again., should be fine. Refill with proper mix of coolant/water.
Most coolants you buy these days are fully compatible with either "old" type. Color isn't really a good indicator anymore, and isn't necessarily "not compatible" with Dexcool.
That used to be the case a few years ago, but nowadays, you'd really have to go out of your way to find a non-compatible coolant on a store shelf.