Create a different color scale for each bar in a ggplot2 stacked bar graph

I have a stacked bar chart that looks like this:

While the colors look nice, it is confusing to have so many similar colors representing different drugs. I would like to have a separate color palette for each bar in the graph, for example, class1 could be use the palette "Blues" while class2 could use the palette "BuGn" (color palette names found here)

I have found some instances in which people manually coded colors for each bar (such as here), but I'm not sure if what I'm asking is possible - these bars would need to be based on palettes, since there are so many drugs in each drug class.

Code to create the above graph:

library(ggplot2)
library(plyr)
library(RColorBrewer)

drug_name <- c("a", "a", "b", "b", "b", "c", "d", "e", "e", "e", "e", "e", "e",
           "f", "f", "g", "g", "g", "g", "h", "i", "j", "j", "j", "k", "k",
           "k", "k", "k", "k", "l", "l", "m", "m", "m", "n", "o")
df <- data.frame(drug_name)

#get the frequency of each drug name
df_count <- count(df, 'drug_name')

#add a column that specifies the drug class
df_count$drug_class <- vector(mode='character', length=nrow(df_count))

df_count$drug_class[df_count$drug_name %in% c("a", "c", "e", "f")] <- 'class1'

df_count$drug_class[df_count$drug_name %in% c("b", "o")] <- 'class2'

df_count$drug_class[df_count$drug_name %in% c("d", "h", "i")] <- 'class3'

df_count$drug_class[df_count$drug_name %in% c("g", "j", "k", "l", "m", "n")] <- 'class4'

#expand color palette (from http://novyden.blogspot.com/2013/09/how-to-expand-color-palette-with-ggplot.html)

colorCount = length(unique(df_count$drug_name))
getPalette = colorRampPalette(brewer.pal(9, "Set1"))

test_plot <- ggplot(data = df_count, aes(x=drug_class, y=freq, fill=drug_name) ) + geom_bar(stat="identity") + scale_fill_manual(values=getPalette(colorCount))

test_plot