merging two different colors with ggplot2 geom_point plot

In cell biology, people often publish images in which one protein is labeled green and a second protein is labeled red, and then they show an image of the green protein only, then the red protein only, and then a "merge" to show where both proteins are present. The merge ends up showing mostly green where only protein 1 is, mostly red where only protein 2 is, and yellow where they both are, and you can see gradients where there are increasing levels of protein 2 in a background of constant protein 1, etc. I want to do the same sort of thing with ggplot, with the x and y axis showing locations of two measurements and the intensity of color indicating the magnitude of the measurement. In the attached example, measurements 1 and 2 are similar over most of x and y coordinates, but measurement 1 is a bit higher from y axis 50 to 100 and measurement 2 is a bit higher from y axis 75 to 125. In my example, graph1 gives me what I want (albeit in blue rather than red), but then I show an unsuccessful "graph2" in which measurements 2 simply overwrite measurements 1.

I hope that with the attached script, this confusing question becomes more clear. Can anyone tell me how to make the desired merge of two different colors (ideally red for 1 and green for the other)? (Any constructive criticism of the somewhat ugly way in which I've created the data frame would also be welcome.)