Assigning colours to each category and displaying the points in a Leaflet Map in R

Question

I have a dataframe [df], just like this:

id  lat         long      category
1   -103.3671   43.0835   0 to 0.5  
2   -103.3634   43.0855   0 to 0.5
3   -103.3684   43.0884   0.5 to 1  
4   -103.3613   43.0835   1 to 1.5

I would like to know how to display these points in a leaflet map with a different colour for each category? I have 5 categories in total.

I tried something like this, but without success:

pal <- colorNumeric(palette = "RdYlBu", domain = df$category)

leaflet(df) %>%
    addCircles(lng = ~long, lat = ~lat, color = ~pal(category), fillColor = ~pal(category), fillOpacity = 0.7, radius = 10, stroke = FALSE)

When defining the 'pal', I get an error: Wasn't able to determine range of domain.

Is there any solution to this? Thank you for any suggestions.