As per my comment earlier you could accomplish this with a formula based conditional format:
To have an email send automatically you could modify the following VBA to your specifications:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim oApp As Object
Dim oMail As Object
If Target.Column = 1 Then 'If the change was made in your entry column
If Target.Value Mod 500 = 0 Then
Set oApp = CreateObject("Outlook.Application")
Set oMail = oApp.CreateItem(0)
With oMail
.To = "YouRecipient@TheirDomain.com, Sample@Gmail.com"
.Subject = "Engine Service Needed"
.Body = "The engine mileage is currently at " _
+ CStr(Target.Value) _
+ " Miles And Needs to be Serviced"
.Display 'You can replace the word display with the word send if you don't want to preview
End With
End If
End If
End Sub
Update From Comment:
For Amber change the formula to `=MOD(A1,500)>0`
For darker orange use `=MOD(A1,500)>300`
And for Red use `=OR(MOD(A1,500)>400,MOD(A1,500)=0)`