If serious about stamp collecting, you need a few basic tools. Perforation gauge, watermark detector (or fluid), colour guide and a catalogue.
For the Deutsches Reich 20pf Germania it could either be Scott 69 which has no watermark and thus would make it ultramarine, or one of the Scott 84 watermarked Lozenge types which has colour varieties of ultramarine, blue violet and light blue.
Color can be difficult to determine, especially through a computer screen, but even in person too.
The scanner may change the color when scanning, the software may change it, and different computer screens may show same picture in slightly different colors.
Even in person, different people see colors differently. Also, environmental effects (sun), water damage to the ink, the amount of lighting used when looking at it, yellowing of the stamp with age, and chemical changing of the ink color can all cause color differences.
Best, though difficult, is to compare the stamp, in person, with another stamp that has been positively ID'd as the variety.