If you've been toying around with the Raspberry Pi for a while, you've likely come across the rainbow coloured splash screen before. There's a good chance you've also seen the more curious version of that, the same rainbow pattern, shrunk down and flashing in the corner of your screen. Here's what they mean. The rainbow screen appears on every boot as a test of the GPU, but if you're not looking at the screen the whole time, you've probably missed it because it flashes by pretty quickly. However, if your screen freezes at this rainbow screen, then you likely have one of two problems.
It could suggest that the
kernel.img file isn't booting, which means there's something wrong with the image on your SD card. To fix this, you can try a new image or try messing around the config page. It could also mean your power supply isn't powerful enough, in which case you'll simply want to try a more powerful supply.
Speaking of power supply problems, if you see the rainbow coloured cube in the corner of your screen, that means your power supply is incorrect. If it's a rainbow, it means your power supply is under-voltage. If it's red, it's over-temperature. Your Raspberry Pi might still continue to work with either of these issues, but you should get the correct power supply as soon as possible.