Heatmap without levelplot

as I'm not sure to understand the coloring levelplot uses, I'm looking for 
another easy way to create a heatmap like this:

library(lattice)
mat <- matrix(seq(1,5, length.out = 12), nrow = 3)
mat[1,2] <- 3.5

my.at <- seq(0.5,5.5, length.out = 6)
my.col.regions <- rainbow(5)

graph <- levelplot(t(mat[nrow(mat):1, ] ), at = my.at, col.regions = 
my.col.regions)
print(graph)