Could anyone provide me with an image of the voltage output of an NXT or EV3 colorsensor. I would like to have the analog output if it's possible, and not the digital output.
Making the assumption here that by analog, you mean something more fine-grained than just the 6 or 7 colors.
Both the NXT and EV3 color sensors have the capability of reading the individual color components. The value read is not a voltage, but just a "percent" of reflected light of each color.
For NXT-G, you can find a block here (the Color Range Block). (see comments)
For EV3-G you can find a block here. The EV3 block requires firmware version 1.06 (there is a bug that prevents it working in earlier versions).