I'm not sure about your island, but my area of the Pacific Northwest was also scraped by the giant ice sheet. Like you, I'm rather devoid of clay. Most of my soil is silty and gravelly.
I have a feeling that gritty/crystal-ly rocks like granite and quartz aren't terribly good for paints. If they were, people would have ground up amethysts and peridot for paint...and they didn't.
Calcium is a great source of white pigments--I wonder if I our big chunky white shells would turn into a nice white pigment?
Lamp black and charcoal should make a nice black.
Woad should make a nice lake blue, but it is very dark--almost like india ink. My daughter painted a watercolor with indigo watercolor, and it hasn't faded in two years, despite being hung on the wall.
Oxidized copper makes a light blue-green color, and I think copper was once found in the area (or maybe just brought in?). I'm pretty sure the area had copper tools for a while, but then switched to mostly bone and some stone tools. Lots of use of cedar!
I wonder what the local Salish Sea peoples used for paint. Did they tribes up in B.C. originally paint their totem poles, or did they simply carve them?
I found
this PDF about totem pole painting and conservation, and it mentions:
“totem poles were painted with a type of fish-egg tempera, consisting of a mineral pigment mixed with a mordant of fresh salmon eggs and saliva. The colors originally were red, black, and green or blue. The red was obtained from hematite, the black from graphite
and carbon, and green/blue from various copper ores common in the
region.”