My 2011 Scott Specialized indicates that J.W. Ferguson & Sons printed the #2902 (perf 9.8 vert.) for Stamp Venturers and that Stamp Venturers printed the #2902B (self-adhesive serpintine die cut 11.5 vert). The stamps were printed using photogravure and the below description, taken from the Errors, Freaks, & Oddities Collectors' Club website, offers an explanation for the color variation.
56. Color variations: Stamp designs used over long periods of time often can be found with differing shade varieties of their basic colors. The properties of the ink components can change, as can the quality and properties of the paper purchased by the printer. Those factors can affect the printed colors. New printing equipment, using the same plates and ink, also can result in shade and color-intensity differences. Unless pronounced, these shades are relegated to minor status and, though avidly collected by many, have little additional value.
The advent of photogravure printing in the late 1960s brought a whole new class of color varieties. Gravure is based on the sequential printing of four basic colors in carefully planned dot patterns, one on top of the other, to produce the impression to the human eye of every color of the rainbow. It produces much more colorful stamps. But it also causes collectors to notice shade differences more than ever before because a small difference in the inking of one color plate often has a dynamic effect on the final colors produced. Expertizers frequently see photogravure stamps submitted as missing-color candidates, when they are only a light print of one of the four basic colors. Thus, there is a range of normal for most photogravure stamps, and a stamp has extra value only if it falls outside that range.