Your right; many of the better known recognition manuals such as Medsger's "Edible Wild Plants". Ewell Gibbons several manuals and Tom Brown's books show the plants as black & white drawings.
There are some pretty good colored drawings in John Wiseman's "The SAS Survival Handbook" and full page colored drawings in Bradford Angier's "Field Guide to Edible Wild Plants".
The Army manual FM 21-76 "Survival" shows plants in color photos. I have a set of edible plant cards for Temperate North America from Life Support Technology, Inc., Manning Oregon 97125. These are excellant quality photos and is a pretty extensive set. They were copyrighted in 1975 and may no longer be available.
I would suggest you might be able to find a course in your area on edible plant identification.