In the future, if anyone needs to determine if a wire is polarized positive or negative, you can use your DVOM to find out. Put the meter range on DC, in a voltage range that exceeds the expected voltage.
Put the red lead (+) on what you think is the positive wire from your supply. Put the black lead (-) on what you think is the negative supply wire. If the display indicates either + or no polarity, the red wire is on the supply positive, and the black lead is on the supply negative.
If the meter indicates a minus sign ( - ) then you have the red lead on negative, and the black lead on positive.