Looking at Display Driver Registry Settings it seems to be possible to set 24 bit color depth for colibri t20: (LDDS = 24, ColorSize = 9).
But it looks like the LDDS settings has no effect on the final result (is there a software limitation on framebuffer?).
Is there a way we can achive 24 bit color depth using Colibri T20 (also using different interface)?
There is a pending issue on this which is documented in our roadmap (Issue 6940). With the issue documentation you find also a workaround. Try to set LDDS in the bootloader config as well.
More Information about config block settings you find on the developer website. You can either use the config block editor to set the ss.ldds or change the setting directly in the bootloader menue.