Dark Coolant means you have a leak some were right??

If you remember I changed my heater core out before winter, replaced all my hoses and flushed my system out. The coolant stayed perfect until now. I noticed it was dark brown instead of green. Its not oily or milky. I looked at my thermostat housing and noticed the gasket is all messed up an there was coolant dripping off it. So I think I am going to go buy a new one, and re flush my system.