Color scale for an ordered variable

I’m trying to make a scatter plot with marker color denoting the data session collection number.

How does one do this?

If I set group = :session, I get non sequential colours (even with, say, color_palette = :blues):

@df dat scatter(:x, :y, yerror = :se,
    group = :session,
    color_palette = :blues)

If I use zcolor = :session, I get sequential colours, but the legend is a continuous color bar.

@df dat scatter(:x, :y, yerror = :se,
    zcolor = :session,
    seriescolor = :blues)

How can I make it an ordered categorical (since there is no session 1.5)?