There can be differences in contrast even within the same brand. Color-wise I haven't noticed any
significant difference in modern lenses, maybe an old chrome barrel Componon (pre-S). More important will be eveness of illumination relative to format, and how this is related to the eveness
of the diffuser or colorhead itself. Diffusers have to be ground to match the specific lens. Longer
than "normal" focal lengths will generally give a more even field. Like I said, I am a bit nitpicky,
but any relatively recent top-end lens by the major mfg (Schneider, Nikon, Fuji, Rodenstock) should
perform admirably.