Years ago I was taught, before computers were in heavy use, to buy red cellophane from an architecture supply store. You could put it over the painting and see value instead of color. A couple of years ago I bought John Lennon red lens flip up sunglasses. I went to Wilson Park and plein air painted by myself. I think I have plein air painted alone only two times. I need to do more of it. Anyway, they really worked. I could not see my palette colors but knew about where they were generally located. I blocked in the attempted mixes on the canvas based on the value I could see. I stopped using the glasses once the values were blocked in then finished the painting. It was really an awesome day though the wind blew so strong it blew my easel over and spilled mineral spirits on my shoe. I enjoyed visiting with people who came by that day. Plein air painting makes for a good day. Sadly I do not do enough of it. I hope we block our paintings in with not so much thought of color like wearing Rose Colored Glasses.