Excel Conditional Formatting Colors

Hello, I wonder if any one can save my day and be a big help.

Im trying to make a conditional format for a cell to turn a specific color when the cell value value of a1 for example is:

- smaller than 0
- between 0 and 15
- higher than 15

I understand I need to select "formula is" under the conditional formatting tab... but what formula do I need to introduce?

Thanks for any help.

and a1 is in a different workbook than the one im working on...