Color-fill ggplot2 maps with a single color

Is there a way to color-fill counties with a NON-ZERO n with a single color (ex. green) AND possibly put a colored dot with a single color (ex. red) for counties with a NON-ZERO n in the middle of the counties?

library("sf")
library("tigris")
library("ggplot2")

me <- counties("Maine", cb=TRUE, class="sf")

me$n <- rep(0:3, each = 4)

ggplot(me, aes(fill = n, label = n)) + 
  geom_sf() +
  scale_fill_gradient(low = "yellow", 
                      high = "red",
                      na.value = "gray90") +
  theme_void()

If you want one fill color for counties with a non-zero n then map the condition n > 0 on the fill aes and set your desired colors for zero and non-zero counties using scale_fill_manual:

library("sf")
#> Linking to GEOS 3.11.0, GDAL 3.5.3, PROJ 9.1.0; sf_use_s2() is TRUE
library("tigris")
#> To enable caching of data, set `options(tigris_use_cache = TRUE)`
#> in your R script or .Rprofile.
library("ggplot2")

ggplot(me, aes(fill = n > 0, label = n)) +
  geom_sf() +
  scale_fill_manual(
    values = c("yellow", "green")
  ) +
  theme_void()

And to add a dot in the center you can use sf::st_centroid:

centroids <- sf::st_centroid(me) |> 
  dplyr::filter(n > 0)

ggplot(me, aes(fill = n > 0, label = n)) +
  geom_sf() +
  geom_sf(data = centroids, color = "red") +
  scale_fill_manual(
    values = c("yellow", "green")
  ) +
  theme_void()