StackExchange got some interesting content at last :D

[Gryzor]

hardware - How did Mode 3 on the Amstrad CPC work? - Retrocomputing Stack

[gerald]

hardware - How did Mode 3 on the Amstrad CPC work? - Retrocomputing Stack

A careful look at the colourMux and VideoControl pages of the GA schematic will give the answer for the color decoding :
The mux only knows about mode 2 and mode 0. Everything else is mode 1.
For the pixel width (in VideoControl) only mode 1 and mode 2 are considered. Anything that is not mode 1 or 2 is mode 0.

[pelrun]

"We'll never know exactly what happens until someone reverse engineers the Gate Array, and while it has been decapped and photographed in April 2016, so far I don't know of any available schematics."


Funny he should say that... 

[robcfg]

Gerald reverse engineered the ga based on our decap pictures and it's pending check against real hardware. You can see the schematics in the hardware/gate array decapped thread.

[Gryzor]

Maybe someone with an account there could let them know