Sentence:  The color work on the postage stamps will not be new to the bureau. In addition to the work which it has done on proprietary stamps, it has mixed and used every shade of color in its other work. Brown was used on the “<WSD>brownbacks</WSD>,” green on the "greenbacks;" different blues on customs stamps and on the backs of bonds. 
Meaning of the word marked with <WSD> tags:  A postage stamp printed in brown ink, used in the 1890s.