Sorry the NXP HDMI adapter used on the BBB only has 16 physical lines for the color… So only 16bit mode. (By forcing 24bit mode you get weird color shifts due to the unpopulated pins.
BTW the exp-bbb-c cape has a 24bit lcd…
You cannot do 24b and say 1920x1080 resolution. There is not enough memory bandwidth. That is why it is 16bits instead of 24bits on the BBB.
If 24b is that critical, then you need to cut the resolution back. Also, the pin mapping is different for 24b vs 16b. All the colors get scrambled. You can look at the TRM for some delightful reading if you need more details on that. IF you set the processor for 24b mode, the colors are all wrong on the BBB.