LMP - I would stain them first then varnish them. If you do wish to make them wild and colourful there are waterbased stains available in vibrant red, blues, greens, yellows etc but I haven't seen purple.
I use a couple of coats of Danish Oil applied with fine steel wool as a sealer, which gives a beautiful smooth finish that fills the grain. Often a few more coats of Danish Oil is enough but if I'm cresting I will finish them off with two thin coats of hard glaze water based gloss varnish as it smells less and dries much faster than poly. If I'm doing more than a dozen I actually prefer to use a dipping tube (very easy to make) as it is super quick and less messy.
The cresting inspiration came from these nesting Mallard hens near my home.