US Special Delivery Motorcycle Colours

I appreciate that the subtleties of colour will be lost in scans and in various screen settings, but I've got 3 samples of the 1927 10c US special delivery motorcycle stamp in 3 distinctly different shades.
The Scott Specialized Catalogue (2008) shows E15 gray violet, E15a red lilac and E15b gray lilac.
How am I to determine which is which?

sc E15 29.Nov.27, grey-violet, perf' 11/10.5
sc E15a 1928, red-lilac, perf' 11 / 10.5
sc E15b 8.Sep.41, grey-lilac

I should have them on covers from various years if that would help?
Regards
Gavin

Collector of "Any philatelic material that depicts a motorcycle, or part thereof, or name of manufacturer, recognizable as being of ITALIAN ORIGIN."

thanks agustanz
are you saying I have them in that order here, top to bottom?

Sorry I can see how that might well look that way... no, it was just to show the different years.
Regards
Gavin

I had a look thought my collection, attached is a scan of a 1923 cover with E12 on it just to show it's color.

The single stamp has a 1934 post mark and should be E15a 1928, red-lilac.

The block has a 1949 post mark so should be E15b 8.Sep.41, grey-lilac

I have pages and pages of these stamps so I probably have E12 from 1927 but not with a clear cancel or on cover so I can't ID it 100%

At least that's a start!
Regards
Gavin

Hi, I need a bit of help determining which color variants I have so I'm resurrecting this thread. I know scans are difficult to aid in pinning this down but I think there's enough color shift to help.

Any feedback is most appreciated, thanks!
Eagle