#6 is a great basic design, but the colors need some adjustment.
The Green arrow and the gold building need to have some gradient. The green colors in #3, #21, or #2 may work better.
The Orange in #21 and #18 has a nice sheen that may work for the building, or a shiny Gold might work too.
I've updated the sample images:
http://imgur.com/a/o5CLaLook at the colors of the buttons and make it resemble a simple classic calculator as much as possible.
Most calculators have only one Red button... It could be a double button (make it larger, like two of the buttons connected) on either the bottom left or the bottom right that would resemble a door.
Maybe all of the buttons (except for the red one) could be either Light Blue with gradient, or Silver, or black with white symbols if that looks better. The window (screen output) could be white (#20) or black (#5), or Light Blue, like the one in the first picture at:
http://imgur.com/a/o5CLaSymbols can include "1", "2", "3", "+", "-","x", "*", "รท", "%", "#", "?", "C", "M" and "=" or any combination.