I do currently have them make value scales using comp. colors. I know you're
right that comp. have more to do with tone than true darkening. THat's why I
brought this up. It seems to me with tempera that the colors more interesting
with the comps even though they are not true shades (although I call them
that). I would say that my students' scales when done correctly get duller and
darker. When I've given them black they tend to get "yucky" colors.
Though I am my no means a painter I've always tried, when I do paint, and
especially when I work with color pencils, to use a darker version of the
local color according to the light spectrum and to darken for shadows I would use
blues, violets, and some hints of complements.