It appears (maybe) that if you want to get striking watercolor, gouache, or pencil outputs from Illustrious - then you might be better off trimming your negative prompt.
watercolor painting, 1girl, solo, close up, face portrait, masterpiece, newest
worst quality, bad quality, low quality
watercolor painting, 1girl, solo, close up, face portrait, masterpiece, newest
<empty>
Why might this be? Well - I think it's because most of the training data that covers scanned sketches, phone pictures of in progress watercolor paintings, etc - are mostly labeled as lower quality.
Left to right - worst quality, bad quality, average quality, good quality, best quality, masterpiece
watercolor painting, 1girl, solo, upper body, <quality tag>
<empty>
The textures we want from watercolor is far more represented in the lower quality labeled data than at masterpiece - which is surely almost all digital art.
We can try adding the negative prompts back, but prompt the various quality tags in the positive anyway,
watercolor painting, 1girl, solo, upper body, <quality tag>
worst quality, bad quality, low quality
When I prompt for watercolor, I'd say I'm looking for something that looks more like the first image rather than the last.