A freckle-tan is the Irish version of a tan. Unlike most other races, the Irish are stereotypically unable to "turn colors" as well as the Italians for example. Thus, we get these uneven dots called freckles. Just as an Italian or Greek will get a bad tan line, or a German or Irish person get a bad burn; a 'freckle tan' is what can sometimes occur for the light skinned peoples. (Almost like a farmers tan but instead of a tan, freckles are present.)
The skin that is not exposed on ones arm will have the same lightly speckled freckles that it has always had... the skin is 'white' at the end of the day. The skin however that was not covered and WAS exposed to the sun will be heavily dotted with very dark freckles.
Thus, when the sleeve is pulled, one will have a distinct freckle line or 'Freckle-Tan'