David:

With respect to the colour of the light, the issue is that a simple lens (such as a magnifying glass) will focus different colours of light to different distances. More complex lenses can and are designed to correct for this - in some cases exceptionally well.

A basic, three or four element enlarging lens is probably not highly corrected for this, but there is some correction built into the design. Higher quality five, six or more element enlarging lenses will most likely be highly corrected for the problem.

So if you use a good quality enlarging lens (Componon, Rodagon, Nikkor) at typical enlargement amounts (full frame, 5 x 7 through 11 x 14 or 16 x 20) this shouldn't be a problem for you. If you are making very large enlargements, you may need to use the higher range APO lenses to avoid the problem.

You need to realize, however, that even if you use the basic lenses, there is another factor to consider. There is a certain amount of depth of focus at the paper plane available to you when enlarging, and any focus shift that falls within that depth of focus will tend to be almost invisible in the result. So, in most cases, stopping down the basic lenses will also help avoid the problem.

I would guess that the target market of Mr. Sussman's book was most likely the hobbyist photographer in the late 1960s, early 1970s. Top quality enlarging lenses were probably fairly rare in that market, at that time.