Firstly unprotect sheet under tools (if protected). The only way i know how to do this is by applying one cell at the top of the screen first with the following formula applied =TODAY(). This cell will the update daily. Maybe put a bold outline on this box and label it in the cell to the left or right 'todays date'.
Now click on the cell with the date of your meeting, reminder etc.
Now go to Format - Conditional Formatting.
You now have up to 3 conditions that you can apply to a cell. There are various options available (colours, fonts, patterns etc) under various conditions (cell value is greater than, less than, equal to etc).
In the first condition select 'cell value is' - 'greater than' - now type in the grid reference of the cell with todays date followed by the subtraction of the number of days you desire before it changes colour. Example below...
This would then change to your chosen colour 5 days prior to the date.
Now assign the necessary colour you wish it to change to under 'format' - colour' and repeat again for further conditions.
It took me a while to work this one out hope it helps you. Any further advice let me know.