What are A and B Ethernet cable standards for?
As Lurick says, there are two color orders, to make it easy to create a crossover cable.
A crossover cable is a cable that makes a direct connection between two network cards, like how you could do with serial cables. The transmit pair is connected to the other card's receive and the receive pair is connected to the transmit on the other card so you have a loop between the two network cards.
On 100 mbps network cards, only 4 wires are used, wires 1, 2, 3 and 6. Wires 4 and 5 (blue pair) was sometimes used for phones or for power over ethernet and the brown pair was not used.
On 1 gbps (and higher) network cards, all wires are used, and the ethernet standard requires network cards to support auto mdi-x , meaning auto detect pairs and recognize if user wants to use a regular cable as a crossover cable.
As for why that order, most manufacturers prefer one of those color orders because it has to do with how the wire pairs are twisted and then the four pairs of wires are twisted together (in cat5/cat5e standard) or separated (in cat6/cat6a and higher).
I'm not sure where I've read but some claimed that arranging them in the order they used made for the least amount of untwisting and least number of wires crossing over other wires when flattened into the connector to be crimped.
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