i may have finally found the reason for yellowing!
It seems as though the UV light output properties of the different manufacturers of bulbs may be the cause!
My kit with ddm ballasts and bulbs were yellowing the rings quite quickly. Culprit? DDM bulbs give off a much higher percentage of UV light than other bulbs! Therefore dependent on the bulb manufacturer you have in your car you might get yellowing quickly or not at all.
Check out these measurements comparing the different bulb manufacturers!
The measurements are for h1 bulbs but similar measurements should apply for other bulbs.