Yes, painting clouds blue happens all over the world :) Here in
Portugal, I also see some students painting clouds in light blue when
the sky is white, or as you described, painting/leaving the clouds
white while painting the sky blue. I remember when I was in 4th or
5th grade, I made a small book about all shapes and colors of clouds,
it was so fascinating to discover the reason for that, that I still
have the book today!
I have an idea, maybe we let our students paint clouds in any color
they want and at the same time we can challenge them to keep a class
daily "graphic diary" about shapes and colors of clouds.