So here is the rule: A hyphen is used to join two words that describe an object, when the descriptive words can't be used on their own.
Example: The blood-stained walls
You can't describe the walls as simply blood, or simply stained. Therefore a hyphen is required.
Example #2: The mushy, green peas.
Both words can describe the peas on their own, therefore no hyphen is required.
So for those of you who like little tricks for grammar and punctuation, I hope this comes in handy.