I recently read Ron Ranson's book on Watercolour Skies and my mix-and-match method seems too timid, too likely to lead to muddy colours. However, I have to say that the final colour is as near as I could have got it. Even now I've scanned it and I've got it onto my web page, when I look out of the window it's still the same grey.
Ron Ranson advises painters to choose the most interesting part of a sky - not necessarily the part that is just behind your subject. With more rain to come, this afternoon's sky is uniformly grey (or ultramarine/alizarin crimson/lemon yellow).