How can I color in one zipcode in a map, RStudio?

I made a zipcode map of dallas using this shapefile: https://gis.dallascityhall.com/shapefileDownload.aspx (Street Files)

dallas_streets %>% 
  sample_frac(1) %>% 
  group_by(POSTAL_L) %>% 
  summarize(geometry = st_convex_hull(st_union(geometry))) %>% 
  ggplot() + ggtitle("Zip Code Map of Dallas") +
  geom_sf(aes(fill = as.numeric(POSTAL_L))) + 
  geom_sf_text(aes(label = POSTAL_L)) + 
  scale_fill_viridis_c(option = "C") +
  theme_minimal()

I want to be able to have the map as greyscale and only one zipcode colored in, please let me know if you can help. Thanks!