When do you put a comma in front of a conjunction?

Always use a comma between two clauses if both clauses are independent clauses.

Otherwise, the Word following the conjunction (part of a different clause) might be mistakenly read as part of the first clause.

For example - there should be a comma before these conjunctions:

His house is built of wood and brick in many varied colors, mainly red and brown, is used in the garden planters.

He was a friendly fellow but a serial murderer, wanted in several states, almost took advantage of his helpful nature.