The R code is fairly simple — just grab the JSON from the server and use purrr to map over the various styles. The returned tibble contains the value returned from the server, along with the label and fill color.
# this builds a dataframe of styles/colors from feature server
feature.info <- jsonlite::read_json('https://services.arcgis.com/cJ9YHowT8TU7DUyn/arcgis/rest/services/AirNowLatestContoursCombined/FeatureServer/0/?f=pjson')
style <- map(feature.info$drawingInfo$renderer$uniqueValueInfos,
\(x) {
tibble(value = x$value %>% as.numeric,
label = x$label,
color = rgb(x$symbol$color[[1]], x$symbol$color[[2]], x$symbol$color[[3]], x$symbol$color[[4]], maxColorValue = 255))
}
) %>% list_rbind()