polygon - Keep customized colors with geom_polygon() - Geographic

R/ggplot2 adding an additional line/polygon to a choropleth map - Geographic Information Systems Stack Exchange

r - How do I change the polygon fill color and border color for SpatialPolygons objects? - Geographic Information Systems Stack Exchange

Chapter 14 Graphing Part II STA 444/5 - Introductory Data Science using R

shapefile - What is the cause of 'tearing' of polygons (artifacts) using R, ggplot and geom_polygon? - Geographic Information Systems Stack Exchange

Custom color in a map(polygons) by ggplot in R and plot a legend - Stack Overflow

Chapter 3 Data Visualization Exploring Data Science with R and the Tidyverse: A Concise Introduction

r - Create hatch color in geom_polygon - Stack Overflow

r - assign colors to geom_polygons where fill = discrete category - Stack Overflow

Chapter 7 Map R Gallery Book

The Grammar of Graphics

r - color polygon boundaries by factor - Stack Overflow