SOLVED:

In System Settings > Colors, make sure the Default Application Color Scheme is not selected. Any other on the list will work. Click Edit Scheme to change colors. Click save and choose a name for your new color scheme. You can now select your new color scheme and click Apply.

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After a Mint KDE update yesterday ( December 19, 2016 ), I lost component editing capability.
I like to choose my own color for such components as: titlebar, inactive titlebar, buttons, selection color, tooltips, etc.
Furthermore, when I click on Help > About KDE in system settings, I no longer see the Plasma version. I guess I'm using 5.7 since the System Settings Handbook mentions Plasma 5.7.

Since using Plasma 4, I've grown accustomed to setting all my component colors. Is there a way one can do that in recent versions of Plasma 5?