The ND has clear glass on the light clusters and relies on the coloured bulbs to give the required, legal amber colour to the indicators. Don’t think silver bulbs would be legal in that case.
I’ve fitted them all around on my NC2, they’re made by a company called SILVATEC, (or at least mine were) they are chromed finish when not illuminated but show orange when powered up. Same as ND, clear front and rear lenses with amber bulbs as standard, mine had the amber side repeater lenses but changed to clear. I think the way they work, is the rear portion of the bulb glass is amber, rest is chrome, so from the front they look shiny chrome, the rear portion (Amber) shines onto the reflector, showing an Amber light. IIRC T20 for main reflectors and T5 for sides on NC, cap less wedge bulbs. Easiest way is find what bulbs are standard and then search for the same bulb number in silver.
TBH, it’s not that hard, first search I did, first result. You don’t tell us what model year, although it’s not going to make any difference in this instance I don’t think. Does the handbook not state which bulb type?
Looks like 585 type wedge 21w front and rear are same type. If you can’t find that bulb in silver there will be an equivalency chart somewhere on the net, as said previously I got T20’s in silver for the NC2. I did also have difficulty getting the right ones but got there in the end!