Locking Cell in workbook

Hi Folks,

I have a requirement where i need to lock the particular cell and i did that using formatting sheet,

Now the thing is when user select/insert/change that cell it is showing error message,

"The cell or chart you are trying to change is protected and there fore read only. To modify...."

Please help me...how to avoid this message.

and i want that protected data(numbers)  should come in light Gray colour.

Thanks in advance for your inputs