It's a book I read years ago, though I suspect I didn't so much as read but rather look at the pictures. But the only thing I can remember from the book is that it had a picture that looked something like this:
The actual picture would obviously more professional than my dodgy rough sketch. And it was most definitely in colour; I think gouache, certainly something bright and opaque.
I am now on a mission to find this book! If anyone could recommend any possibilities, I'd be very grateful.
Because if I can't find it, then the alternative must be that I travel back in time and illustrate an edition of Heidi myself.