use black instead of grey

Author: cpsievert

NEWS.md

@@ -8,7 +8,6 @@
 
 * Default continuous color scales (i.e., the `options()` `ggplot2.continuous.colour` and `ggplot2.continuous.fill`, which inform the `type` argument of `scale_fill_continuous()` and `scale_colour_continuous()`) now accept a function, which allows more control over these default `continuous_scale()`s (@cpsievert, #3827)
 
-
 * A newly added `geom_function()` is now recommended to use in conjunction
   with/instead of `stat_function()`. In addition, `stat_function()` now
   works with transformed y axes, e.g. `scale_y_log10()`, and in plots

R/scale-hue.r

@@ -177,5 +177,5 @@ okabeIto <- c(
   "#0072B2",
   "#D55E00",
   "#CC79A7",
-  "#999999"
+  "black"
 )