Great Britain Scott 356c Help

Great Britain 356c is the Wilding 2d Queen Elizabeth with graphite line on back. I find mention of a Light Red Brown in my catalogues, but the left stamp is bothering me a bit. I'm thinking sunlight fading, but look forward to your comments.

Left stamp is the probable faded stamp (Mint, full gum on back)
Middle stamp is a "normal" color (Mint, hinge mark)
Right is a scan of another "normal" back. All three stamps have line on right side when viewed from back.

The colour was changed, from red-brown to light red-brown in October 1956 (St. Edward's Crown watermark). That is why you find the reference to 'light brown.' The righthand stamp should be 'normal' in the sense that the normal colour for it is light red-brown. That middle stamp also looks very much like a light red-brown. Although I cannot be sure from the scan, your remark that all stamps have a graphite line on the back clinches it. They only come in the light red-brown colour.

This already should give you the answer to your question. The left-hand stamp is almost red it is very faded.