"开放式画风的" can be translated as "with an open/free-style painting approach" or "characterized by an open/free painting style."
Here are a few example sentences to illustrate the usage:
1、 The artist is known for his works with an open-style painting approach, which allows for great creativity and spontaneity.
(这位艺术家以其开放式画风的作品而闻名,这些作品展现了极大的创造力和即兴性。)
2、 Her paintings are characterized by an open painting style, blending different techniques and colors seamlessly.
(她的画作以开放式画风为特点,将不同的技巧和色彩无缝地融合在一起。)