Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/STMicroelectronics/STM32L4S9/LTCD/L1CKCR#0x0
LTDC Layer Color Keying Configuration Register
Color Key Blue value
Color Key Green value
Color Key Red value
https://github.com/cmsis-svd/cmsis-svd-data