Update {lighten,darken}_color() documentation and add tests

R/color.R

 #' @description Produces a linear blend of the color with white or black.
 #' @param color_hex A character string representing a hex color
 #' @param strength The "strength" of the blend with white or black,
-#'   0 low to 1 high.
+#'   where 0 is entirely the original color and 1 is entirely white
+#'   (`lighten_color()`) or black (`darken_color()`).
 #' @name lighten_darken_color
 NULL
 

man/lighten_darken_color.Rd

 \item{color_hex}{A character string representing a hex color}
 
 \item{strength}{The "strength" of the blend with white or black,
-0 low to 1 high.}
+where 0 is entirely the original color and 1 is entirely white
+(\code{lighten_color()}) or black (\code{darken_color()}).}
 }
 \description{
 Produces a linear blend of the color with white or black.

tests/testthat/test-color.R

     expect_error(full_length_hex("#00000Z"))
   })
 })
+
+
+describe("lighten_color() and darken_color()", {
+  it("errors if strength not in [0, 1]", {
+    expect_error(lighten_color("#123", 9))
+    expect_error(darken_color("#123", 9))
+  })
+
+  it("returns original color if strength = 0", {
+    expect_equal(lighten_color("#808080", 0), "#808080")
+    expect_equal(darken_color("#808080", 0), "#808080")
+  })
+
+  it("returns entire blend color if strength = 0", {
+    expect_equal(lighten_color("#808080", 1), "#FFFFFF")
+    expect_equal(darken_color("#808080", 1), "#000000")
+  })
+
+  it("returns midpoint if strength = 0.5", {
+    expect_equal(lighten_color("#000000", 0.5), "#7F7F7F")
+    expect_equal(darken_color("#FFFFFF", 0.5), "#7F7F7F")
+  })
+})