Can't Seem To Highlight Particular Cells When Certain Sum Reached

I am trying to create a special workbook for my asphalt project. After inputting all of the day's truck information, I want to add up the total tons and then for the program to highlight which truck puts the total tonnage at or slightly above 500 tons. I'm not sure if "isolating is the right term to describe what I wish to do or not, but my whole workbook will be based off of similar actions like what I'm requesting. Here is a portion of my sheet:

Truck #Tonnage
1 21.93
2 22.84
3 23.21
4 24.89
5 24.8
6 25.01
7 24.88
8 24.44
9 23.14
10 24.53
11 22.03
12 22.61
13 24.9
14 24.69
15 24.85
16 24.81
17 24.82
18 22.55
19 22.18
20 24.52
21 22.49
22 24.81

And so on.....