Navigate to Administer -> Settings -> General, search for the Instance color setting and look at its description. It says

"Hex value. for color for the instance title banner. Will only be used if an instance title is defined. If no valid value is given, the default value is: #000000. (requires a page reload)."

There should be no dot after the "Hex value" I believe. Also it should say "It will only be used". I'd also omit the last dot.