SUMMARY
Custom color controls for Background, Active Icon, and Inactive Icon do not become enabled/disabled immediately in "Window button colors" page, when turning the corresponding options on/off in "Window buttons" page.

STEPS TO REPRODUCE
1. In System Settings, Appearance, Application Style, Widget Style, select the QtCurve style and then click Configure.
2. In the "Window buttons" page, check the boxes for "Use custom background colors" and "Use custom icon colors". (Initial values for these options, upon opening the Configure dialog, were both unchecked.)
3. Go to the "Window button colors" page.

OBSERVED RESULT
All of the color picker controls are disabled (matching the initial state of the corresponding checkbox).

EXPECTED RESULT
The color picker controls should become enabled (matching the current state of the corresponding checkbox).

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Kubuntu 18.10
(available in About System)
KDE Plasma Version: 5.13.5
KDE Frameworks Version: 5.50.0
Qt Version: 5.11.1

ADDITIONAL INFORMATION
After checking the boxes in step 2, clicking OK on the Configure dialog, and then clicking Configure again to reopen the dialog, causes the enabled/disabled state of the color picker controls to update to match the aforementioned checkboxes. Similarly, unchecking those checkboxes will not disable the color pickers until the Configure dialog is closed and reopened again.