Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/STMicroelectronics/STM32U599/LTDC/LTDC_L1CACR#0x0
LTDC layer 1 constant alpha configuration register
constant alpha These bits configure the constant alpha used for blending. The constant alpha is divided by 255 by hardware. Example: if the programmed constant alpha is 0xFF, the constant alpha value is 255 / 255 = 1.
https://github.com/cmsis-svd/cmsis-svd-data