Green is the complementary colour of red. It goes well with medical accessories and doesn't shout for attention. Green, which provides a high-contrast environment, reduces eye fatigue and make bright red blood splashes less conspicuous. Also,we can find green colour as neutral even if we observe in spectrum.
(Earlier the colour used was white...the combination of bright operating lights and an all-white environment led to eyestrain for the surgeon and staff. By the 1950s and 1960s, most hospitals had abandoned white and apparels were selected in favor of various shades of green)