Digital BW, The Print

This is a B&W printing 101 question. Please humour me.....

A student of mine is photographing digitally then desaturating the
images in PS. She is then using the history brush to bring specific
colour spots back into the image. The colour picker shows that the B&W
portions of the image are absolutely neutral. Yet there is a magenta
cast in the mid to highlight areas and green tones in the mid to
shadow areas.

Can anyone suggest a workaround that will avoid these cross curves yet
retain the colour spots? (Colorspace: Adobe 1998, Epson 4000, Epson
enhanced matte and Epson EM profile, Epson driver.)


Thanks

John Malcolm
New Zealand