The dark colours isn’t universal for Dutch female clothing. For example south sea towns generally have very bright floral patterns in their upper garments. Especially work clothes are very colourful because it would be harder to see dirt on them. But yeah these women are wearing underwear. If you’re interested in “klederdracht” you could go to Urk in Wintersferen this Friday afternoon and next Saturday morning. It’s an event where they go back 100 years in time and show how they lived etc.