What Do You Think It Is #510 Or #510A

Kevin, I originally replied to Bluegrassriver2 to note how a particular color shade doesn't necessarily indicate rarity. Eventually, I refined my replies to a more accurate answer of which is the rarest, the #510a (brown yellow).

I mistakenly claimed that a #364 was more "rare". I was trying to use it as an example to strengthen my point that color shades are often shared amongst types and issues. Thereby pointing out that looking for a particular shade of color is not very helpful without using references so that one can identify a rare stamp. I was trying to take into context that Bluegrassriver2 seems new to this aspect of stamp collecting.

You replied that the "#416a brown - yellow" was the rarest. You are correct that the brown yellow is the rarest color for a 10c 1908 series Franklin.