You only set offsetCell1 and offsetCell2 during your first match. You need to include those two lines in your Else statement as well.

    If c.Value = strInt Then

'Check if this is the first match (new range hasn't been filled yet)

    If MyRng Is Nothing Then

'Fill new range with cell

        Set offsetCell1 = c.Cells.Offset(-1, 0)
        Set offsetCell2 = c.Cells.Offset(1, 0)
        Set MyRng = Application.Union(c, offsetCell1, offsetCell2)

    Else
'Join new matching cell together with previously found matches

        Set offsetCell1 = c.Cells.Offset(-1, 0)
        Set offsetCell2 = c.Cells.Offset(1, 0)
        Set MyRng = Application.Union(MyRng, c, offsetCell1, offsetCell2)

    End If

If colorTap doesn't return any value then it should be a Sub, not a function, and whenever you call a method and don't capture a return value, you should not put parentheses around the arguments:

colorTap Range(Range("B4").Offset(i,-1), Range("B4").Offset(i,lastCol) 

Unneeded parentheses cause the argument to be evaluated before being passed to the method, which might be the cause of the problem you're seeing.

FYI no need for Select here - you can just use With x.Interior