The black rule says: The solution is easy, but not "easy".
The red rule says: The solution is simple, but not "simple".
The golden rule says: The solution is straightforward, but not "straightforward".
What is the solution?
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All three of those words, Easy, Simple and Straightfoward, translate to "einfach" in German (The German flag being described by the colored rules), which I presume is the answer. Also, not sure if intentional, but a rule can be a type of line (see http://printwiki.org/Rule_Line or https://en.wikipedia.org/wiki/Ruled_paper), potentially meaning the "rules" are lines on the flag.