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.
Depends on what programming software you are using. The education versions of the NXT and EV3 software having data logging features that may be what you are looking for.
The education versions of NXT and EV3 programming software are available for purchase from www.education.lego.com
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).