Originally posted by goddo31 TwoUptons is correct; the scan should be done as greyscale, thus eliminating any colour cast. In my opinion the lab should have done that for you. However it is easy to do in software once you have the scanned images.
I don't agree with that myself. XP2 and Kodak CN-400 are "color process black & white film" They are actually color film, but the amount of color information captured is VERY small.
But.. if you scan in color, and manually control the sensitivity and expand out the histogram for each color as much as possible, you get a lot more information.. and from that you can then use a mixdown layer to very carefully blend the color information into the monochrome. Because each color has very different sensitivity, some will have better shadow detail, some better highlight detail, and some better mid-level detail, so this "mixing" process can give you fantastic control over the levels of detail on the final "mixed down" black & white. It's a lot like being able to blend 3 copies of the same photo, one taken with a red filter, one with a green filter, and one with a blue filter. (or, one with a cyan filter, one with a magenta filter, and one with a yellow filter.. with a little photoshop-fu, you could mix & match all 6 of those, and with some masking only allow certain layers to affect it etc..)
When it comes to scanning, especially where the data set might be limited to shades, to get as much control over the conversion process as possible.
If I'm going to do a sepia, I do that after I'm happy with my mixed-down greyscale, using a hue/saturation layer, ticking the "colorize" box, and then tweaking the color to my liking.