@Quiest: I did some research at
U-Boot, came up with this:
First bootscreen (Dark Gray) is compiled into U-Boot MTD image.
Second splashscreen (Green) is in both U-Boot image and Linux image.
U-Boot starts a framebuffer, dumps first screen to it. It then checks for Firmware upgrades through FTP, SD etc, if none are present it starts a second framebuffer and dumps second splash screen to it. Then it switches frame buffers and loads Linux
When Linux loads, memory from first frame buffer is released and Linux starts a frame buffer, loading it with it's own copy of second image and switches to it.
Then U-Boot terminates and that memory is released to pool.
Basically we'll have to recompile both U-boot source and Linux source with our bitmaps in.
U-Boot states only 225 distinct colors allowed, otherwise will crash, reason not specified
- or I got tired of reading the manual
So, yes it's possible to replace the boot screens, not likely immediately.