Plastic parts (pickup covers; knobs, switch and tremolo-arm tips; etc.) come in many colors, and from many different manufacturers. So there is a lot of color variation, and this it true sometimes even when the color is the "same". As an example not all black parts will match each other, though you might think they would. There can be subtle differences not just in the color, but in the finish of the plastic as well.
The challenge for us is to stock plastic parts only in colors where customers can reasonably expect to find the other parts to match. Fender is sort of the authority here - they offer complete sets of plastic in a number of colors - so we sell only their knobs/covers/tips as accessory items.
Several vendors that we buy from offer parts in various colors. For instance, Suhr offers Strat pickups in white, parchment, aged green, and black. But they only have the covers, not matching knobs and tips, so we would only order their Strat pickups in white. If a customer wants something other than white, then we would refer them to the Fender accessories.
Callaham is another vendor with options, as they offer trem arms with parchment, black, or mint green tips. But we only stock them with parchment.
A little history lesson regarding mint green: For several years in the early 1960s, Fender used a pickguard material that would age to mint green. This seems sort of iconic to many people today, but at the time it was surely a headache for Fender, and they pretty quickly replaced the plastic with another type that did not turn green. The early '60s knobs, pickup covers, and switch/trem arm tips, however, did not turn green over time, they yellowed instead.
When Fender released the '62 Reissue Strat in the mid 1980s, they made mint green pickguards for use on this model, since the original 1962 Strats came with the unstable plastic. Fender also made what they call aged white (yellowed) knobs and covers to mimic the aged look of the early guitars. Fender has never used mint green for knobs, pickup covers, or switch/trem arm tips.
So again, our primary concern when stocking any sort of plastic is this: Can we get knobs, covers, and switch/trem tips to match? We stock items from our vendors in colors that match what Fender offers.