Richarlison, 21, signed for Watford last summer for £13m from Fluminense when current Everton manager Marco Silva was in charge at Vicarage Road.
The Brazilian forward made a big impression on his arrival to the Premier League, scoring five goals in his first 12 appearances.
A £50m deal would see Richarlison become Everton’s club-record signing, overtaking the £45m the Toffees paid for Gylfi Sigurdsson last summer.
Elsewhere, Serie A outfit Roma have made a £31.3m offer to Bordeaux to sign Everton target Malcom, according to Sky in Italy.
Sky in Italy understand the player has already agreed personal terms with Roma after a positive meeting between his agents and sporting director Monchi on Thursday.
Sky sources reported earlier this week that Everton were interested in signing the Brazilian.