This is one of the most misidentified stamps on the part of dealers and collectors, compounded by a lack of specificity on the part of the Scott catalog.
Like R97, R13 comes in the same 3 distinct shades: blue, milky blue, and ultramarine. However, unlike the former, Scott does not have a listing for all 3 colors of the latter. Personally, I think this needs to change, but that’s a tough sell with the editors.
With R13c, the blue and milky blue shades occur in approximately the same frequency; neither one is more valuable than the other. Only the ultramarine is scarce.
Since Scott does not list a milky blue shade for R13c, many people think they have an R13e when they encounter the milky blue shade.
When you see the 3 shades side by side, the difference jumps off the sheet. Ultramarine has a magenta component that pushes the hue towards the purple/red end of the color spectrum.
Here is an image that I frequently send to sellers on eBay. It is a scan of all 3 stamps together on a card.
From left to right:
- R13e ultramarine
- R13c milky blue
- R13c blue