Complex sentence: It replaced the "courtly style" of about 1440 to 1474 during the southern Netherlands reigns of Philip the Good and Charles the Bold, which had consisted of works in primary colors and "wooden, clumsily painted stock figures".
Split into simpler sentences: It replaced the "courtly style" of about 1440 to 1474 during the southern Netherlands reigns of Philip the Good and Charles the Bold. That mid 15th century style consisted of works in primary colors of "wooden, clumsily painted stock figures".