I am not certain it's possible to directly glean the original color from a baked-in flat file.
But, I think you can make an educated guess...
Duplicate the layer and set the duplicate layer's blend mode to
Multiply. Then zoom in and watch the pixels, duplicate the multiply layer repeatedly until you see a pixel (probably in the center of a stoke away from an edge) which is not getting darker. That is probably close to the original color.
It's not as easy to tell in this gif.. but the pixel I chose was one in a sea of changing pixels. The area of color change was greater than what the gif may actually show. That particular pixel remained the same color for 2 iterations of the layer duplicating. That's enough for me to assume it's the original color. If I keep duplicating the multiply layer it's going to eventually start getting darker.