Possibly too dry between waterings. Drying out only once or twice will cause leaves to look quite beat up. Bougainvillea come from a wet-dry climate, not exactly a monsoon climate, but a climate that has a fairly dry 3 month long dry period. They are opportunistic in how they deal with drought. They drop leaves easily, they can sit dormant for as much as 4 or 5 months, but the minute water is available they will start growing. They do not require a dry rest. Hence the term opportunistic. Unlike some trees that must have a dry rest, Bougainvillea will grow anytime water is present. But in order to survive drought, they loose leaves rather easily, not putting energy into keeping leaves that would get lost during drought. So while looking ugly, I doubt this is a serious condition.
If you were too heavy handed with the sumo cakes, an excess of nutrients can cause the reddish color. If this seems a possibility, take off the cakes and maybe a little of the surface soil, replace with fresh media. Then give water with no fertilizer until it is safe to put it outside. (knowing Minnesota, it might not be until June). But if you feel you were not overdoing the fertilizer, then don't remove the fertilizer.
I also noticed that under LED leaves will often take on atypical coloration. This could also be the source of the colors.
So we have 3 possibilities. You need to decide which is the cause. All 3 are either just tolerate the ugly leaves until new ones come in spring.
You can remove the ugly leaves if they really bother you. There will be a flush of new growth, usually pretty quickly after defoliation, 3 to 6 weeks.