I have a few questions:
-how long did this show run for?
-was it really a talent incubator?
-was it shot in B+W only?
-are the reruns available today?
Finally, was the late George Scott ever on Playhouse 90? 
It ran on CBS from 1956 to 1961. It was always in black and white. As far as reruns, I’ve never heard of them being played on TV, although some of them are available on VHS and maybe DVD. A lot of the episodes were later remade as feature films and some of the episodes that those films were based on have been released on video over the years. And yes, George C. Scott was in the episode “Target for Three”, which aired on October 1, 1959. Some other famous people who were on the show at one time or another (this may help answer your question about the show being a talent incubator, although some of these actors were certainly famous before “Playhouse 90”) (also, this isn’t the complete list of all the actors to ever appear on the show, just the ones whose names I recognized):
Jackie Coogan
Charlton Heston
Tab Hunter
Vincent Price
Kim Hunter
Jack Palance
Keenan Wynn
Eddie Cantor
Peter Lorre
Boris Karloff
Shirley Jones
Red Skelton
Ralph Bellamy
Lloyd Bridges
Ethel Barrymore
Louis Jourdan
June Lockhart
Imogene Coca
Robert Preston
Lew Ayres
Nanette Fabray
John Carradine
Harpo Marx
Mary Astor
Piper Laurie
Anne Bancroft
Billie Burke
Raymond Burr
Melvyn Douglas
Zsa Zsa Gabor
John Drew Barrymore
Burl Ives
Patricia McCormack (who played Hellen Keller in “The Miracle Worker”! Most people know her as the evil little girl from The Bad Seed. I also never knew that The Miracle Worker had appeared in any format other than live theatre before the movie version with Patty Duke.)
Teresa Wright (Annie Sullivan in “The Miracle Worker”)
Mickey Rooney
Mel Tormé
Claudette Colbert
Paul Henreid
Royal Dano
Barbara Hale
Lee Remick
Hedda Hopper
Art Carney
Johnny Carson
Carol Channing
Joan Blondell
Patricia McCormack (Here she is again! This time in “Child of Trouble”. Hmm…)
Ricardo Montalban
Richard Kiley
Polly Bergen
Sylvia Sidney
John Cassavetes
Michael Rennie
Errol Flynn
E.G. Marshall
Ernie Kovacs
Carl Reiner
Sterling Hayden
Patricia Neal
Tony Randall
Jack Lemmon
Joseph Cotten
DeForest Kelley
Maureen O’Sullivan
Farley Granger
Patricia McCormack (!)
Ben Gazzara
Robert Vaughn
Keenan Wynn
Lee J. Cobb
Vera Miles
Robert Stack
James Mason
Hope Lange
Paul Newman
Joanne Woodward
Charles Bronson
Buster Keaton
Chico Marx
Betsy Palmer
Geraldine Page
Dick York
Fess Parker
Patricia McCormack (#4)
Dennis Weaver
Yvonne De Carlo
Angela Lansbury
Rod Taylor
Barbara Bel Geddes
Patricia Hitchcock (Alfred’s daughter)
Robert Loggia
Martin Balsam
Rip Torn
William Shatner
Rod Steiger
Jeanne Crain
Elizabeth Montgomery
Cliff Robertson
Jackie Gleason
Jack Klugman
Theodore Bikel
Eartha Kitt
Roddy McDowall
Beatrice Straight
Ray Walston
Melvyn Douglas
Buddy Ebsen
George Balanchine (in “The Nutcracker”)
Bonnie Bedelia (Clara in “The Nutcracker”)
Peter Graves
Dyan Cannon
Art Carney
Micky Dolenz
Leslie Nielsen
Burt Reynolds
Maximilian Schell
Dean Stockwell
George Takei
Jason Robards
Eileen Heckart
Tommy Kirk
Richard Beymer
Suzanne Pleshette
Lee J. Cobb
Patricia McCormack (#5)
Dale Evans
Ricardo Montalban
Martin Landau
Claire Bloom
Dana Andrews
Rita Moreno
Mike Nichols
Arthur Kennedy
Robert Redford (as a Nazi Lieutenant)