For the Silvertec indicator bulbs you don't need to add resistors at all as they are just normal type bulbs just with a special silver coating. The coating means that when they are housed in your headlights you don't get the orange glow or fried egg look as in an unlit state the bulbs are silver in colour, ...turn the indicators on though and they flash orange just like any other indicator bulb.
Perfectly legal as far as I'm aware and passed through MOT's with them fitted.