Word of mouth spoilers aside, yes, anybody who had seen mention of it in the newspapers or magazines, or heard about it on the radio would have known. The March 15, 1938, edition of the New York Times ran an article entitled "52 New Pictures Listed by M-G-M", with a sub-headline "Color Pictures Planned":
"The studio, which last week signed up with Technicolor, plans to make three films next season in the new process. They are 'Northwest Passage,' with Robert Taylor, Spencer Tracy and Wallace Beery, 'The Wizard of Oz,' which will be a Mervyn LeRoy production with Ray Bolger, and Jules Verne's 'Twenty Thousand Leagues Under the Sea.'"
An AP article that ran shortly after (including in the Hartford Courant on March 16, 1938) also mentioned the planned movie being in color, under the headline "MGM Plans Technicolor Film of 'Wizard of Oz'":
"Metro-Goldwyn-Mayer announced today it will make 52 pictures, including 30 in the big-budget, big-star class, during 1938-39.
"The list of films is topped by three in technicolor: 'Northwest Passage,' 'The Wizard of Oz' and 'Twenty Thousand Leagues Under the Sea.'"
Both articles appear to be based on some kind of press release issued by MGM.
Occasional items about the progress of the film ran in the months leading up to the film's release that mentioned it was a Technicolor production, such as an AP blurb that ran in the February 5, 1939, edition of the Pittsburgh Press that ran under the headline "Morgan Dons Green Beard as 'Wizard'":
"Frank Morgan, the mustached comedian, will wear a beard also, in his role as 'The Wizard of Oz.' It will be green for Technicolor. In the course of his role, Morgan appears in several guises as the wizard, who can be hairy or clean-shaven at the wave of his hand."
As the release date approached, many more newspaper items appeared which mentioned it being a Technicolor film. One such article was in the August 9, 1939, edition of the Washington Post under the headline "'Wizard of Oz' Takes Rank With 'Snow White'" with the sub-headline "Fantasy a Gem of Technicolor Magnificence":
"This department agrees with the private tip recently dropped in a New York mailbox by Oscar Doob, of the home office executive staff of Loew's, Inc., that there is only one earlier picture with which 'The Wizard of Oz' reasonably may be compared. That picture, of course, is 'Snow White and the Seven Dwarfs.' Yet, beyond the fact that each visualizes a fantastic fairy tale and dedicates itself exclusively to the extravagances of fantasy, the two are not greatly alike. Yes, they are, too, in one respect. Each presents a superb example of Technicolor photography and the complete adaptability of natural color to the depiction of this sort of screen entertainment..."
Upon its release, notices and reviews of the film mentioned it being filmed in Technicolor. The August 17, 1939, edition of the New York Herald Tribune contained an article entitled "'Wizard of Oz' Opens Today":
"Metro-Goldwyn-Mayer's Technicolor production, 'The Wizard of Oz,' opens today at the Capitol Theater..."
The next day, the opening paragraph of movie critic Frank G. Nugent's review of the film in the New York Times mentioned it was "drawn in the brightest colors".
From the time the film was released, print advertisements regularly mentioned the film was in color, such as one in the August 18, 1939, edition of the Pittsburgh Post-Gazette which promoted it as "A Triumph of Technicolor!". Magazines such as Redbook, McCall's, and Movie Mirror ran features about the movie in either their August or September 1939 issues, mentioning the movie was in Technicolor. Original 1939 lobbycards displayed in movie theaters also called it "Metro-Goldwyn-Mayer's Technicolor Triumph!" There were also radio promotions for the film, such as an episode of the series Leo Is On the Air (a.k.a. Hollywood Is On the Air) which mentioned the film being a "Technicolor extravaganza" (just after the 1 minute mark).
So, yeah, anybody who had read even a bit of press about the film, or heard any promotion of it on the radio in advance of going to the theater knew it was going to be a color film.