Please read the other posts in this forum about this issue, I've tried to explain in detail several times.
In short, the way twisted pair wire works, you twist the two wires into a 'pair', then you send a signal (plus and minus voltages) down that pair of wires. As long as you keep the same signal on a pair, they tend to cancel each other out (basically) and they don't generate a large field causing interference with adjacent signals.
The way ethernet works, one pair is transmit, the other receive. You want to keep those seperate, if you don't, you will get the wrong signal coupled on the other wire and it will disrupt communications. Now, to make that work, we transmit on one twisted pair, and receive on the other. When you terminate the cable such that a signal is not on a pair, it does not have that self cancelling effect, and it disrupts the other signal.
Also keep in mind that different pairs are different lengths, since the twist rate is not the same on each pair (another method to keep the signals from coupling with the other pairs). So if you use one wire of the blue pair, and one of the green, those two wires will physically be different lengths. Hence the signal will not arrive at the other end at the same time on the both wires (delay skew).
Frankly it isnt that critical at 10 Mbs, I can make ethernet work at 10 Mbs on barbed wire for a short distance (well, almost). However, as you start increasing the speed to 100 Mbs AND as you expect full duplex (transmit and receive at the same time) AND as you increase distance about about 20 feet it becomes very important.
Thanks for asking why, it's always good to understand the reason, we don't require it just because we prefer certain colors :)
Hope that is what you need, happy holidays!
It is only my opinion, based on my experience and education...I am always willing to learn, educate me!
Daron J. Wilson, RCDD