You will note as you acquire more and more stamps from that period, you will find shades and color varieties. All of them due to many reasons. Whether it is multiple printings, fading of ink, sun damage, etc.
The color purple especially you will find many shades, usually not catalogued.
For this series however, Michel does list 4 types. which are design varieties. Note: the 15c stamp only has designes I,II, and III
Here are some of mine (Scott 113, Michel Type II)

Michel Design Types

Here below is a set of the purple 25 cent stamps from the next definitive series in 1922. Again, note the colors. In this case, Scott does give a minor number to the dull violet shade (versus Magenta). Scott 151 and Scott 151a

