With the Reaper 5 theming implementation, it's basically the way both tcp_bg.png (track unselected) and tcp_bgsel.png (track selected) graphic files are tinted that defines the panel backgrounds. If they are both fully opaque, the panel will be tinted exactly as these files, even if the Preferences>Appearance>Track Control Panels>Tint track panel backgrounds option is ticked.
Things become different if there is a more or less amount of transparency in them : if there is an area fully transparent, it will get exactly the tint of the track color choosed.
__________________
DAW: Ryzen 3700X|Asus X-470Pro|32Gb|2 SSD512(M2)+1024|W10Pro(64)|RME Fireface UCX+ADA8200
Soft: Reaper|Cubase 10|Emulator X3 & several other VSTis (2 bridged)...
Gear: VMK-188+|MPD32|ME30P|Korg 05R/W|Roland D110|Yamaha TX802|Pre-MIDI stuff...
|