Change standard error color for geom_smooth

Your (understandable) mistake is to think that you should be changing the color rather than the fill. The standard error shadings are made with geom_ribbon essentially, and they are a 2d area, so the "color" they are "filled" with is determined by fill, not colour.

Try:

geom_smooth(aes(...,fill = variable))

where variable is the same one you map to colour elsewhere.

If you have multiple groups you simple define color = Vars and group = Vars within ggplot(aes()), and then additional aes(fill = Vars) within geom_smooth(aes(fill = Species)). (Based on answer here)

Dummy example:

# Make confidence intervals the same color as line by group
ggplot(iris, aes(x = Sepal.Length, 
                 y = Sepal.Width, 
                 group = Species,
                 color = Species)) + 
  geom_point() + 
  geom_smooth(aes(fill = Species))  # additional `aes()` referencing to confidence interval as a `fill` not as a `color`