Ok. Perfect. Thank you so much

This is where phrasal verbs meet colloquialism and idiom.... "Mellow out" comes from 1960's California colloquial slang. "Don't trip on that man! Mellow out!" - "Chill out" would be a somewhat more modern version of the same thing. Maybe he's mellowed out means: Maybe he has calmed down (another phrasal verb). Maybe he has ceased being agitated, he has calmed himself (down). It is true that "washing up" in the UK means to wash the dishes, but "wash out" in America does not mean the same thing. Here we just say "wash the dishes". "Wash out" has various meanings, including: "To become lighter or less vivid in color" - "She dyed her hair pitch black last week but now it looks washed out". It can also mean a degree of failure: "It was a wash-out!"

I think the 'out' here is just adding emphasis to the 'mellowed'. There are a number of phrases using out, for example: - 'chilled out': "looks like he's chilled out now", meaning he's now fairly relaxed - 'decked out': "he's completely decked out his car", meaning his car has all gadgets, decorations, etc. - 'maxed out': "I've maxed out my credit card", meaning I've reached the credit limit on my credit card (oops!)