You may notice that when you turn on or reset your Amiga a few different colours flash by. These actually mean something. Here you go:

RED     Bad ROM
YELLOW  CPU Exception Before Bootstrap Code is Loaded
GREEN   Bad Chip RAM
BLACK   No CPU 
WHITE   Expansion passed
GREY    CPU Passed

Constant white is failure of CPU.

Or as taken from the Amiga 4000 Service Manual:
Passed Test      Light Grey     Initial hardware configuration tests passed.
                                Initial system software tests passed.
                                Final initialization test passed.

Failed Test      Red            ROM Error - Reseat or replace
                 Green          CHIP RAM error (reset AGNUS and re-test)
                 Blue           Custom Chip(s) Error
                 Yellow         68000 detected error before software trapped it (GURU)

The system performs the following test sequence:
1. Delays beginning the tests a fraction of a second to allow the hardware to stabilize.
2. Jumps to ROM code in diagnostic card (if found)
3. Disables and clears all DMA and interupts.
4. Turns on the screen.
5. Checks the general hardware configuration.
   If the screen remains a light gray colours and the tests continue, the hardware is OK.
   If an error occurs, the system halts.
6. Performs checksum test on ROMs.
   If the system fails the ROM test, the screen display turns red and the system halts.

One Blink     ROM Checksum failure
Two blinks    RAM test failed
Three Blinks  Watchdog timer failed
Four blinks   A short exists between two row lines or one of the
              seven special keys (not implemented)
Five Blinks   Your keyboard has become nuclear. RUN!