Automated way to create polygons from coloured map

Question

I have a scanned map that has areas of different colours. I want to create polygons for each area and would like to avoid doing it manually (tracing over the shapes).

Is there a way to do this? I do not have ArcGIS - am using QGIS.

I have tried various feature extraction options to no success (probably choosing the wrong options).

I have tried creating a layer of just the e.g. blue sections with everything else transparent, importing that, and then polygonising this - but I just get a polygon the size of the whole area.

I have also tried to follow How to digitize a raster automatically (Processing toolbox no longer exists, options I tried in Orfeo unsuccessful); and https://docs.qgis.org/2.2/en/docs/training_manual/forestry/stands_digitazing.html (polygons whole area).

I have something like this

except the colours aren't quite so clear - it's a shaded map overlaying satellite imagery, so there's lots of tiny "gaps", but I can fill these without too much hassle if needed.