Conditional formatting

Put 66 into a cell by itself (I used A1).
Then select Format/Conditional Formatting
change the 2nd dropdown to "greater than or equal to"
in the 3rd box enter =$A$1 (or whichever cell you decide to use)
format/patterns to the color you want

for b), I think you are out of luck if you want to have a "selectable color" You can set this manually in CF, but you cant do it automatically

For (b), you could create a userform that contans a grid of command buttons.
Set the back color of each button to the color you want for the selected cells. Code the command button to color the selected cells

Basic example:

When the user form is initialized, set the back color of all command button

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For each button in the userform, code it like so:

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You can determine the correct RGB color value by simply opening up the format cells dialog (Ctrl + 1 keys).
Fill Tab > More Colors > Custom
Move the color selector (in the large color grid) to the color range then move the slide (narrow strip to the right) to the desired color.
Note the RGB values for the selected color. These are the values to use in the code.

See attached example.

Excel-Help_Color_Cells.xls