The communication protocol is documented at https://xboxdevwiki.net/PIC#The_LED
The Xbox's front LED is driven by the PIC16LC63A (SMC) motherboard component. Upon startup, a set of 4 time slots are reserved for it and looped in succession. The first three slots consume 180 ms (milliseconds) and the last 200 ms for a total of 740 ms per full LED cycle.
The time slot used for the first color in a cycle depends on when (specific timing TBD) the SMBus set led command was executed relative to the PIC startup, therefore we cannot easily predict which color gets the additional 20 ms via emulation at the moment; what we can assume however is each full cycle takes 740 ms.
An LED cycle can be interrupted (specific timing TBD) but the currently shown color is displayed for its full time slot before honoring the new cycle request.