One option would be via conditional formatting ?
If you want A3 to be shaded light green
when the value in it is 10 or more
Select A3
Click Format > Conditional Formatting
Make the settings under Condition 1:
Cell Value Is| greater than or equal to| 10
Click Format button > Patterns tab > Light green > Ok
Click OK at the main dialog
If you want the fill color to appear in a cell other
than A3, say in the adjacent B3, you could try ..
Select B3
Click Format > Conditional Formatting
Make the settings under Condition 1:
Formula Is | =A3>=10
Click Format button > Patterns tab > Light green > Ok
Click OK at the main dialog