Panel colorize to match desktop color scheme

In the taskbar Appearance configuration, there is a check box option "Colorize to match the desktop color scheme" that has effect when the "Enable background image" option is enabled. On my system, when I select the default.png background image and then enable the colorize option, the color is always a "sky blue." Regardless of what I change in KControl, I can't use any color other than that sky blue. I want to use the default.png background, which has a gradient look, but I want a darker panel. From where is this color derived? I tried browsing the code but am clueless. Thanks. :) Darrell