Hello all, this is my first post and I already have a question.

I'm originally a sign painter and have done murals and illustrations on signs (oil based paints)but here I am with my first (sort of)serious acrylic painting and I don't know how to achieve a certain colour...Could some of you kindly have a look at this painting by William Turner?
http://www.book530.com/paintingpic/0822h2/The-Blue-Rigi-Lake-of-Lucerne.jpg
My hubby loves this painting so I'm planning at painting my version to surprise him but I seem to be stuck with the blue in the sky and mountain. It looks like an almost pure ultramarin in places but that is color is way to intense to use. When I tone it down I get away from the original too much. Kind of looks like the painter has gone over it with an almost transparent layer of white? Is there a transparency agent for acrylics or is hat not necessary? How would you as an artist tackle this? Sorry if these appear to be stupid questions and thank you for being patient.
Lotti