Viridis is a great palette to use because it shows contrast between low and high values very well, and is color-blind friendly. Below, I've written code that can be copy and pasted into R to change the color palette of your visualization to Viridis:
Here's how to add the viridis color palette to R using scale_fill_gradientn:
scale_fill_gradientn(colors = c("#440154", "#414487", "#2A788E", "#22A884", "#7ad151","#fde725"), breaks = c("1,2,3,4,5,6"))
scale_fill_gradientn lets you use any colors you specify in your gradient. In this case, we specifies the colors of the viridis gradient!