Here I go again. Please forgive me for asking...
What is the rainbow color box when it boots? Is it a photo or is it system generated... and why is it on there at all? I don't mind it, but if I can change it to something more meaningful I'd like to.
Its not a "splash screen", its a GPU test. The GPU puts four colored pixels on the screen, then stretches and blends (rescales) them to fill the whole screen.
The rescale function of the GPU is also used when using composite vide output, the GPU simply rescales everything that would appear on an HDMI screen to a resolution suitable for composite video.
You can't change it to "something meaningful" because as I said its basically just four pixels!
You also cannot remove it, is part of the "firmware" of the GPU.