thank you for taking the time to read my question.
I Have 2-Pin 12 Volt Small Fans (4 of them) that I would like to use for a little project of mine. I was going to use this item 
to hook up all the fans into one input connector, but I quickly got stumped on something very small.
I wanted to connect the top female molex connector to a male molex connector as you can see here: 
I looked up and saw that on the second picture, the yellow wire is the 12 Volt wire while the red wire is the 5V wire. On the contrary, in the 4 way splitter, the red wire is unknown (12 Volt or 5 VOLT?) and the yellow wire is the fan speed control wire.
Since I wanted to hook up the female molex connector (the isolated connector on the 4 way splitter) to the molex cable in the second picture, I have no clue which wire goes where. Does the yellow wire on the second picture molex cable connect to the red wire on the 4 way splitter, and can the 4 way splitter even carry 12 volts? I assume so since it is made for CPU fans, which are all mostly 12 Volt.
This is my question:
Which wires do I need to align together when I connect the two molex cables (EX: yellow with red, black with black), and could re-assure me by telling me "yes that red wire on the 4 way splitter carries 12 volts.
Best Answer
Yellow is 12V, red is 5V, black is the nominal 0V. On your 12V fan, red is going to be +12V, so yes, connect yellow Molex to red fan. And black to black.
Connecting red to red will produce the fan running very slowly on 5V, or not at all if that's too low a voltage to get the fan rotating.