@Informatix: Yes you are correct, I meant A for Alpha. The markers are the default ones in OSMDroid. But that is not really the problem. The RGB values do not match.
I want to use the RGB value to tell the difference between river and not river. Looking at the two sets of RGB values, they are very different although photoshop says they are the same. That is what I don't understand. Is this still due to the alpha value? I don't think so.
Just for info. I generate the map in OSMDroid, tap two positions, make a screenshot, save it, start a new activity, read the screenshot file into an imageview, calculate the position of the two markers and read the pixel color. The maths for the navigation can only be started when I have this problem solved.