Why do some Brown Pelicans have orange or reddish parts on their bills?
After doing some reading at the Birds of North America webpage on Brown Pelicans.
It does mention orange on the bill outside of the breeding season.
Brown Pelicans have a greenish gray bill once they are older than 24 days which becomes gray-brown in their first year. At around 12 to 14 months of age, yellow will begin to appear at tip and on the sides. At 16 to 19 months of age, the bill becomes gray-green-yellow with some orange, the orange becomes more distinct at 29 to 37 months.
It is difficult to define the plumage of a Brown Pelican during their first 2 to 3 years; they acquire definitive plumage between the ages of 3 to 5 years old.
Here are the descriptions of their bill in Definitive Basic and Alternate plumages:

In Definitive Basic plumage, proximal end of upper mandible pinkish
orange, distal end buff yellow over pinkish orange, and nail at tip
buff yellow. Lower mandible pinkish orange, except middle section gray
or mottled gray. Proximal end of gular pouch reddish orange (P. o.
californicus) or green-gray (P. o. carolinensis); distal end dark
gray-green. As breeding season approaches, colors intensify; bill
becomes brightest 4–6 weeks before pouch.
In Definitive Alternate
plumage, upper mandible bluish pearl-gray and pinkish on proximal end,
buff-yellow-orange-red over bluish pearl-gray on distal end, and
bright buff-yellow on nail. Lower mandible pale yellow on proximal
third, bluish pearl-gray elsewhere; yellow at maximum intensity when
young being fed; may serve as target for begging nestlings.
Colors
fade with onset of incubation through prebasic molt; upper mandible
gray proximally, buff-yellow and orange over pearl-gray distally, and
buff-yellow on nail; distal two-thirds of lower mandible gray;
proximal end of gular pouch faded yellow-gray (P. o. californicus) or
gray-green (P. o. carolinensis); distal end dark green.

It appears their bill does retain the orange after breeding although it may begin to fade and it does mention how the colour intensifies during breeding.
Reference:
Shields, Mark.(2014).Brown Pelican (Pelecanus occidentalis), The Birds of North America (P. G. Rodewald, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America: https://birdsna.org/Species-Account/bna/species/brnpel