I have a bonsai ficus ("ficus elastica bonsai" I think). For the last 2 weeks, some of the leaves are getting a little black/red (copper color). I took a picture. It seems, but I'm not sure, that this is due to direct sun exposure: for several years, this bonsai grew inside my house. Within 2 weeks, but only I would say 4 times, I placed it outside on the terrace in the sun. NB: it could be due to the fact I have pruned some of its branches to give it a shape (some sap had flowed), as you can see in the picture, 2 weeks ago.
Why do its leaves change from green to a copper color? Does this mean that a dangerous biological process is taking place for the bonsai? Is it possible to heal it?