Okay, here is one way, using Conditional Formatting, assuming you only want to highlight the entries.
I put the following headings in:
B2: Incident
C2: Open Date
D2: Resolve Date
Then starting in row 3, I added values.
Select the entire region (in the example B3: D7), then go to Format > Conditional Formatting.
In the dialog box on the left, from the dropdown change "Cell Value is" to "Formula is". Then in the box to the right, put this formula (be sure to get the dollar signs correct):
=$D3-$C3<=30
and click on the Format button on right, and choose green fill.
Then click on the ADD button, and do the same for the second condition, except put this formula in the box:
=AND($D3-$C3>30,$D3-$C3<=60)
and click Format button,a nd choose yellow fill
Then click the ADD button again, and in the dialog put this formula
=AND($D3-$C3>60,$D3-$C3<=90)
click the Format button, and choose red fill.
Change some of the dates and see what happens to the color.