The output from the pixel shader in Direct3D is four floats, normalized 0-1 for each channel (RGBA). But what happens if these floats haven't been normalized correctly and are above 1?
If you're target surface is a floating point variety (HDR rendering for example) then it just outputs the color; during later processing your tone mapper decides how to move this full range down to 0..1 for display. If the surface is a typical RGB display it gets saturated (forced to a range 0..1).
In any case, it's a very easy test to do to see how it looks in real life =)