DailyUI #04 Calculator

I didn't expect so many questions to arise from such a simple exercise. Design questions of the day:
1. What font to use for displaying numbers?
Samantha Zhang has a very nice article on this topic that I'd suggest you read: http://blog.invisionapp.com/best-free-fonts-for-numbers/. I chose Varela round to make the calculator more friendly.
2. What shape and size should the buttons have?
I started with a round shape, influenced by the iconic Braun calculator, but ended up with square ones, 80px x 80px to fit my 4 column grid.
3. What button configuration to chose?
You'd expect that after so many years we'd have settled with one, but no there are configurations for all the tastes.
4. What colors to chose?
I started with the colors that Dieter Ram used for the white Braun calculator, but I ended up color coding them differently.. Black for numbers. Colors for operators. Grey for the rest.