Have you ever seen a green horse, purple apple, or blue rose? I have. Do I need to go see a doctor about my eyesight? No. I saw a green horse, purple apple and blue rose in art work created by myself or others.
Why would someone create art work with those strange colors? Because they can! Because as an artist you can create anything with any color of your choosing. How fun is that?
My grandchildren love to draw horses. They do not care what color the horse is. They might decide they want to make a pink horse with purple polka dots or stripes. They create their horses with what ever art supplies are available . They do what feels right to them at the moment. My grandchildren have been raised around horses. They know that horses are not green. But the do not let realism get in the way of creating art.
I think some artists get caught up in their artwork being very realistic. They worry about all the colors being exactly as they are in nature. They want their work to look like a photograph. As an artist, when you try to be extremely realistic you often get very stiff and tight in your work. Painting becomes more of a job than a fun artistic process.
I had a teacher that once told me that if I wanted my work to look like a photograph, then I ought to just go buy a camera and be done with painting. Why did the teacher tell me that? Because I was stuck on being realistic.
If you want to loosen up a little try using arbitrary colors in your art work. Think like a child. Have fun. Make the sky brown, the trees red, the grass purple. Have a creative and colorful day!