More than 6.6 billion of "United States three-cent banknote" stamps were printed during 1870-1881. They were printed in three different companies, National B.N.C., Continental B.N.C., and American B.N.C., which was using two differently engraved image carriers. Color variation in inks of all these stamps was examined via non-destructive qualitative fluorescent spectroscopy, using EAGLE III mProbe - Energy-dispersive X-ray Fluorescence Spectrometer. It was found that ink in stamps contained calcium, barium, lead, zinc, iron, chromium, silica and aluminum. Relative levels of the metals in the inks varied, which probably resulted in color variation between those stamps. Stamps from 1878 and newer did not contain zinc in inks. Also, the relative level of metals in individual series of stamps varied, namely in levels of Ca, Ba, Pb, and Zn. Non-printed areas of those stamps were also analyzed for metals content. It reveals that every company used paper with different metals: in National paper was found Ca, in Continental Ba and Zn, in American - Ca, and in American re-engraved paper Cu, Fe, Mn, Mg, Si and Al were found. This indicates that each company was using different paper, which could lead to color variation between stamps originating from different companies.