You can figure this out if you have an ohmmeter. First, pull the RCA plugs out of the amp. Then, disconnect the wires from the cartridge at the cartridge end. Pick the RCA plug that is marked right. It may be marked right with an R, or it may be coded red, or it just may be marked one way or another, like with a black plug instead of silver. If there is no marking, just pick one RCA plug and call it "right," and call the other one "left." Put one of the ohmmeter leads on the center pin of this (right) RCA plug. Then try each of the cartridge wires until you find the one that is continuous with the center pin of the right channel. This cartridge wire should be red. If it isn't, you should change it with the red wire. Then put the first ohmmeter lead on the outside shield of the right RCA plug. Once again, measure cartridge wires until you find the one that is continuous with the right shield. This one should be green. If it isn't, change it. Then go to the left plug. Put one ohmmeter lead on the center connector, and find the cartridge lead that is continuous. This one should be white. If it isn't, change it. Put an ohmmeter lead on the shield of the left RCA plug, and find the cartridge wire that is continuous with it. This should be the last one. This should be blue. If it isn't, change it. Now you have the cartridge wires properly coded, and you just hook them up according to the color code on the cartridge.
If you don't want to move the cartridge wires around, just make a list of the wires by color as they come up. The standard code is right signal = red, right ground = green, left signal = white, left ground = blue, but you can write it however it comes up for you. Then you know which leads should go to which cartridge pins. Hook the cartridge up this way, and all will be cool.
As far as I know, there is no particular "official" wire assignment, other than the standard color code that I have described. I am sure that all the Pioneers are probably uniform, but this way you will figure it out without any specialized Pioneer knowledge. I have made many similar mistakes, and the ohmmeter has saved me over and over again. If you don't have one, get one. You can pick one up at Radio Shack or Lowe's for less than $30.00, and you will use it over and over again.
Hope this helps! Good luck from the old constructor.