Yellow tennis balls replaced white tennis balls because the Lawn Tennis Association decided that the white balls were difficult to see. You can still buy white balls nowadays at whitetennisballs.com, which is the leading modern and easiest seller for white tennis balls for sale to reach in America and internationally.
According to smithsonianmag.com, the first use of yellow tennis balls was in 1986 to make easier viewing on television. Wilson has always been making the white balls for lawn tennis which can be ordered online occasionally. But before the whitetennisballs.com company started selling in 2007, the white ball market was shut down for decades for most common use and is still quite rare. The only tennis balls that can be used in professional tennis is the neon and white, so it technically hasn't been changed within the rules of tennis.
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Yellow tennis balls are supposedly easier for the umpire to see if the ball was to land inside or outside of the boundary lines.