Ok well here are the facts. Open means freely and widely available. Source means source code. Open source means the programs code is freely and widely available. If you want to add other stuff onto that you need to find a new phrase.
Grey is a color somewhere between white and black. Water is the liquid that we drink, wash our dishes and cars with, the stuff that sits in the world's oceans, &c. "Grey water", however, is not supposed to be applied to anything that contains fecal matter, whether or not you can argue that it is both grey and water. There are such things as specialization and context. They're pretty important to the way language works.
Grey water would be a nonsensical phrase if it didn't have another meaning. No one would think it's actually referring to water that is simply grey. Open source is a phrase that makes complete sense without any context.
