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I see browned tips and pits.
I do not know what causes the pits.
Browned tips are common when plants are not getting the right water. Begonias need plenty of water, but good drainage so the soil is not saturated. If the soil stays too wet the roots do not get the oxygen they need.
The leaves are being nibbled or sucked by some type of aphid, insect crawling of creeping / flying, Aphids would very rarely make as large a round hole in any leaf as you show so it indicates to me it's a larger type of insect maybe like an earwig, (brown with pinchers at it's rear) these come out at night and hide away in dark places in day light hours as they don't want predators like birds ect to snap them up for lunch.
The change in colour on the foliage could be shock / stress, as indicated by Diana when she mentioned lack of water, it could be a sudden change in temp, light, cool evenings but doubt it in your area but, never the less I feel there has been a sudden change in conditions, watering, over or under feeding, over crowding, wrong soil, but whatever it is, it has happened more recently than just after planting.
The best way to try trap the insects is to go out at night with torch / flashlight and search both under the leaves and inside the flowers, also for earwigs, you fill a small pot with shreaded paper say from your paper shreader, stick the filled pot upside down onto a garden cane about 1-2 feet above the plants, in the morning, go take the pot to a paved area and empty it out, crush any trapped earwigs you see run for their lives or drop the paper from pot into bucket of boiling water. replace fresh paper to pot and place on cane, repeat this till problem goes.
IF you find it's strange growing conditions then try rectify this and the plant should recover, here in UK, as soon as the very cold nights arrive, the leaves on my Begonias turn darker BUT keep in mind there are some Begonias that have darker leaves but the do start off as green leaves when newly germinated.
Hope you soon find success and all goes well.
I have the same problems. I think its more of an indoor growing condition that doesn't have enough air movement. Its some kind of viral/bacterial infestations and when I move my plant to outside with a more airy environment. The browning and damaged leaves seems to stop and the new growth have not have such a problem but then the temp outside is a little low for my area and I have to return the plant inside and the same problems came back too.
Looks like a sempervirens (wax). The pitting looks typical of any wax begonia in fairly hot sun without adequate water when this happens. Simply picking any ugly, damaged, and old leaves off often cause the plant to put out brand new leaves in no time (a few days at least).