scale_color_gradientn() not producing the correct output

Question

I am having trouble using ggmap + ggplot to create a plot with a custom grid. Here is the plotting code that I am using with some quickly generated sample data. It successfully generates a plot but ignores my custom colors in scale_color_gradientn(). Any suggestions would be greatly appreciated!

library(ggplot2)
library(reshape2)
library(ggmap)

Lon <- runif(100, -100, -92)
Lat <- runif(100, 34, 45)
Conc <- runif(100, 1, 8)

JECeGRIDLoc <- c(39.2865, -96.1172)

PlotModel1 <- data.frame(Lat, Lon, Conc)


### What I need ###


al1 = get_map(location = c(lon = JECeGRIDLoc[2], lat = JECeGRIDLoc[1]), zoom 
= 06, maptype = 'satellite')
al1MAP = ggmap(al1)
al1MAP + geom_tile(data = PlotModel1, aes(x = Lon, y = Lat, fill = Conc)) + 
scale_fill_gradient(limits=c(min(min(PlotModel1$Conc), 
min(PlotModel2$Conc)), 
max(max(PlotModel1$Conc), max(PlotModel2$Conc))), low = "yellow", high = 
"red") +
scale_color_gradientn(colors = c("purple", "red", "orange", "yellow", 
"green")) +
xlab("Longitude") +
ylab("Latitude") +
ggtitle("Jeffrey Energy Center (2012): \n eGRID Model Dispersion Area")


### Additional example (does not work) ###


ggplot(data = PlotModel1, aes(x = Lon, y = Lat, fill = Conc)) +
geom_point() +
scale_color_gradientn(colors = c("purple", "red", "orange", "yellow", 
"green"))

Answer 1

Swap fill to colour and it works

ggplot(data = PlotModel1, aes(x = Lon, y = Lat, colour = Conc)) +
geom_point() +
scale_color_gradientn(colors = c("purple", "red", "orange", "yellow", 
                               "green"))