I saw this movie (in color) approximately 15 years of ago on German TV. I don’t think it was a German movie; I can’t be sure which language the original is shot in, however.
Part of the plot how I recall it (some details might be wrong): A successful, well-known painter dies, but his friends manage to keep his death a secret and start to forge his painting to sell for profit. The main forger begins a relationship with a woman who I think was the death painter’s girlfriend/wife (I think I remember a sex scene in the atelier between the two). I don’t remember much esle except that the scam is discovered (unsurprisingly) at some point.