The color of the translucent selection seems to be tied to the background color for selected text, which can be set in the Control Center.
For anyone interested, the translucent selection effect can be enabled through the "Appearance & Themes > Style" dialog in Control Center. On the "Effects" tab, change the "Rubberband effect" option to "Make translucent". This works for Debian, but I don't know if the effect's implementation is distro-specific or if it's present in the standard KDE.