Can my customers see the renders?

Yes. A paid plan covers renders shown to the users of your product, including when your product is a platform or widget your clients embed on their own sites, as long as the images are served by your product and the clients receive neither the files as a product of their own, nor your API key, nor API access. Reusing the one render you already hold for two clients with the same vehicle and color is fine: the library restriction is about offering renders on their own, not about serving your product efficiently.

What a plan does not cover is reselling, sublicensing or proxying raw API access, or handing renders to others as an asset pack, feed or dataset they use outside your product. An agency building for several unrelated businesses puts each on its own account, or runs the integration as its own product. Ask support before an arrangement that hands files to a third party.