How to change highlight color of windows on taskbar in KDE Plasma 5?

Various colors can be changed via System Settings.

• Under Workspace Theme, make a note of the themes that are active in Look and Feel and Desktop Theme.
• Go back to the opening screen of System Settings and click on Colors.
• Under Application Color Scheme, click on the name of the scheme you have as active in Look and Feel.
• Below the list of themes, click Edit Scheme.
• Ensuring that you haven't changed the Color set from Common Colors, scroll down till you come to Hover Decoration.
• Click on the existing color to open a color chooser palette. Here, select the color you want and click Okay.
• Click Apply and Save. At this point, to avoid overwriting the original scheme, give another name for your scheme.
• Close the window.
• Now, click on the scheme you created and then click Apply once again. That's important!

Note that:

• you haven't really changed the entire scheme. Your changes are stored in a small file in ~/.local/share/color-schemes with the scheme name you gave earlier and a .color suffix.
• more than one visual aspect may be affected by the change you make. It will not be limited to just the taskbar.

As you get more familiar with things you can make many more changes to suit your visual needs.

Please accept this updated answer for a newer version of KDE, I'm not sure from which version it started, but this applies to KDE Plasma 5.18.5 (which I'm running), via GUI:

1. There's now a Colors section in System Settings - go there.

   To find it:

   • Go to to System Settings > Apperance > Colors

   Or

   • Search in `System Settings for "Colors".

2. Highlight (with mouseover) your current color scheme. An edit button will appear (if you hover on it it'll say "Edit Color Scheme")

3. Click on the edit button. It'll open up the KColorSchemeEditor color scheme editor.

4. Make sure you're in the Colors tab (should be the default tab opened).

5. Scroll to look for "Hover Decoration" and then click on the color box.

6. A color selection tool should appear. Select your desired color.

7. Click on OK - it should auto-close the color selecton tool

8. In KColorSchemeEditor click on the Apply button. It shouldn't auto-close the window but you can close it manually by clicking on the Close buton.

9. Then, back in Colors, click on the Apply button. Again, it won't auto-close the indow, but you can close it manually by clicking on the close button.