Plants primarily use light for photosynthesis, and different wavelengths (or colors) of light are absorbed by chlorophyll and other pigments. Chlorophyll mainly absorbs blue (around 430-450 nm) and red light (around 640-680 nm) effectively. However, it absorbs green light (around 500-550 nm) the least effectively, which is why most plants appear green to our eyes; they reflect, rather than absorb, this wavelength.
In summary, the color that absorbs light the worst for plants is green light.