If you check the footnote below the listing for #17 in the Unitrade catalogue there are many shades of this stamp. So it could be quite difficult to discern a specific colour.
Quote:
There were 26 printing of No. 17... 1) nine printings in 21 shades... 2)eight printings in 16 shades... 3)nine printing in 24 shades...
The catalogue describes the perfs associated with each printings, but I wanted to draw attention to the known existence of the
many shades in this issue. I have 5 copies of #17. 3 copies are definitely in the "17b brown (and shades)" category but they are all a little different. 2 of my copies I feel are of the red lilac group, but again both are noticeably different. I can post a scan of them all if you are interested.