Using Gimp, make a copy of your texture file and rename it to "(charactername)_blend.png" (minus quotes obviously) and then open it up.
If your texture doesn't have one already go into layer > transparency and add an alpha channel. Next, use either the magic wand tool or the select-by-color tool to select all of the parts of your texture that you want to recolor, and then invert the selection and delete everything else, this will leave you with just the bit you want to recolor.
(please note that it may take a few tries to get everything perfect when deleting certain colors, play around with different amounts of threshold to make sure none of the bits of the color you want to change color are transparent).
Finally, select everything and use tools>color tools>saturation and drag it all the way down to make your greyscale image, save it but don't close out of gimp and play with it in SRB2.
Playing with the brightness and contrast (Accessed by going to tools > color tools > brightness and contrast) will affect how light/dark your texture's final colors will be in the game so be sure to tune it to where you feel is good enough.