Plants are totally a great source for many pigments! Vincent isn't going to recommend this because setting aside trivia level knowledge, he wouldn't know how to extract it or if there's any plants of that type around.

...Also oranges tend to come from mixing yellow and red pigments which are often from bugs and only the occasional... um, anyway!

"A gaslight, maybe. They're all around. Touch them, and you could find yourself experiencing visions. Hearing the thoughts of others."

Vincent has been suffering at least one of those; probably not the thoughts one.

It's the touch and have visions about an object. This is also why he's avoiding direct contact with anything right now.