That's the flag during the Republic of the Seven United Netherlands. We call this flag the statenvlag (state's flag) and it's used by the republicans. The "usual" orange-white-blue (prinsenvlag or prince's flag) was used by the orangists (wanting Orange-Nassau dynastyto be Dutch king). Today however, Dutch nationalists and fascists want the oranje-blanje-bleu back as national flag. I don't know why the modern-day flag uses these darker shades.