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Turns out it was redshift (higher values of blue in a B-V means redder - doh!)
For large scales, the redshift is very obvious and is referred to as photometric redshift in professional circles. It affects both the emission lines and the spectrum itself (remember redshift means the light waves are stretched - all the lightwaves) It's a few orders of magnitude less precise than measurement via spectral line shifts but doen properly it is accurate to 0.1 z (where z = redshift value).
A plot of the galaxies in B-V and R-I show a strong and steady shift from 0.01 z to -.15 z and I was event able to identify a second cluster of galaxies in the field. Abell 1631 at 0.05 z and a closer one at 0.015 z.
Cheers
David
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