I bought a multimeter with attached leads. I would think red is positive but when I take a reading it only works if I use black as positive. Was it assembled wrong? There's nothing in the manual. It's a greenlee dm-20.
Every dignified multimeter uses red for positive and black for negative.
In addition to that, when you're measuring volts, you can just switch the inputs. It won't go wrong, you'll just get a negative value.
If you suspect it might be assembled wrong, just check it with a voltage source of which you know which pole is the positive and which the negative. Connect the positive to the red lead and the negative to the black lead. If the reading is positive, red is for positive. Otherwise, it is indeed assembled incorrectly, and you should use red for negative.
One observation- the POSITIVE output lead for the OHMS function is usually the BLACK lead on an analog multimeter, and usually the RED lead on a digital multimeter.
So if you measure the conductivity of a diode with an old-school analog multimeter you may have to swap the leads around to get it to conduct (or not conduct).