How to change the axis tick values when using scale_fill_identity

General
2

You can adjust the tick marks on the x axis with scale_x_continuous and y should be scale_y_continuous

eg1 <- expand.grid( blue=seq(1, length.out=13, by=2), 
                    red=seq(1,70, by=10))
eg2 <- within(eg1, mix <- rgb(green=0, red=red, blue=blue, maxColorValue =70))

library(ggplot2)
ggplot(eg2, aes(x=red, y=blue)) + geom_tile(aes(fill=mix), color="white") +
  scale_fill_identity() +
  scale_x_continuous(breaks=c(10,20,30,40,50,60,70), labels=c(1,2,3,4,5,6,7))

Created on 2022-11-02 with reprex v2.0.2