diff --git a/R/color.R b/R/color.R
new file mode 100644
index 0000000..512f3e2
--- /dev/null
+++ b/R/color.R
@@ -0,0 +1,40 @@
+assert_color <- function(color, null_ok = FALSE, na_ok = FALSE) {
+  color_pattern <- "(?i)^#[a-f0-9]{3}$|^#[a-f0-9]{6}$|^transparent$"
+
+  checkmate::assert_flag(null_ok)
+  checkmate::assert_flag(na_ok)
+
+  name <- deparse(substitute(color))
+
+  if (is.null(color) && isFALSE(null_ok)) {
+    cli::cli_abort(
+      paste0(
+        "{.strong {cli::col_red(name)}} cannot be {.strong NULL}."
+      )
+    )
+  }
+
+  if (!is.null(color)) {
+    if (is.na(color) && isFALSE(na_ok)) {
+      cli::cli_abort(
+        paste0(
+          "{.strong {cli::col_red(name)}} cannot be {.strong NA}."
+        )
+      )
+    }
+  }
+
+  if (!is.null(color) && !is.na(color) &&
+      !color %in% grDevices::colors() &&
+      !checkmate::test_string(color, pattern = color_pattern)) {
+    cli::cli_abort(
+      paste0(
+        "{.strong {cli::col_red(name)}} is not a valid color code. ",
+        "It must contain a hexadecimal color code or one of the ",
+        "values in {.strong {cli::col_blue('grDevices::color()')}}."
+      )
+    )
+  }
+
+  invisible(NULL)
+}
diff --git a/README.qmd b/README.qmd
index e9ac25e..786c56f 100644
--- a/README.qmd
+++ b/README.qmd
@@ -1,25 +1,3 @@
----
-editor: source
-output-dir: "."
-post-render: "R/.post-render.R"
-format: gfm
----
-
-```{r}
-#| label: setup
-#| include: false
-
-library(beepr)
-library(cffr)
-library(codemetar)
-library(groomr) # https://github.com/danielvartan/groomr
-library(here)
-library(prettycheck)
-library(readr)
-library(rutils) # https://github.com/danielvartan/rutils
-library(stringr)
-```
-
 # prettycheck
 
 <!-- badges: start -->
diff --git a/codemeta.json b/codemeta.json
index 12f2599..4a8208a 100644
--- a/codemeta.json
+++ b/codemeta.json
@@ -298,7 +298,7 @@
     },
     "SystemRequirements": null
   },
-  "fileSize": "80.735KB",
+  "fileSize": "81.768KB",
   "citation": [
     {
       "@type": "CreativeWork",