I am trying to install an outside motion activated light fixture, there is a box with a yellow wire that did not have power... I found a white wire with power and joined it to the yellow wire...and after installing the fixture (white wire to white wire, black wire to black wire) my fixture is not turning on... uld it be because of the two kind of wires??
No, the wire colors wouldn't matter if they were wired correctly. First, if you see a yellow wire, that is not a conventional wiring setup. You need to find a black wire (hot) that is switched, and a white wire (neutral) that is at ground potential. There should also be a green or bare copper ground wire. On the fixture, black goes to black, white to white, and the ground wire to the metal fixture if there's a ground connection, or if the fixture screws to a metal box, the ground can be connected to the box instead.
This does not sound like a conventional setup that meets code. There are two ways to connect switched power to a box. The first is to do the switching before the box, in which case you should see one black, one white, and one bare copper wire (ground) coming out of the box. You would hook your fixture as follows: black to black, white to white, and green and bare to the box. The second configuration has the wires from the switch joining the power wires (black, white, ground) in the box. In this case, the black wire should always be hot, no matter what position the light switch is in. The wires from the switch should be the same as from the circuit: black, white and bare copper (ground). What you describe sounds very suspicious, unsafe, and not to code. Unless you are very familiar with safe electrical practices and residential wiring, I suggest you call in an electrician to troubleshoot the problem and correct it.
