How come grass isn't blue or pink, but apparently, it is according to this scientist it is every color but green. I also got told by my teacher that grass is only green and no other color. So I am confused, but with an answer please try and be simple.
Plants contain chlorophyll for harvesting energy from sun-light.
From Wikipedia: Chlorophyll:
Chlorophylls absorb light most strongly in the blue portion of the electromagnetic spectrum as well as the red portion. Conversely, it is a poor absorber of green and near-green portions of the spectrum, which it reflects, producing the green color of chlorophyll-containing tissues. Two types of chlorophyll exist in the photosystems of green plants: chlorophyll a and b.
This can be seen from the absorption spectrum of chlorophyll a and b:
So your teacher's statement "grass is only green and no other color" is somewhat over-simplifying. More precisely you can say: grass is a mixture of cyan/green/yellow/orange color (wavelengths from 490 to 610 nm).
Plants require the absorption of light to sustain them. This means that their leaves contain chemicals that absorb light and turn it into chemical energy that fuels the plant, in a process called photosynthesis. The most common of these photochemicals is called chlorophyll. It absorbs most colors of light in the visible spectrum except green, so the light that passes through a leaf or reflects off it has a strong green tint.
It is not a coincidence that grass is green. Of course chlorofyl does absorb little green light but that is hardly an explanation. I can't find the reference right now but there was a study that argued that it serves to balance the power generation over the course of the day. In the morning and evening the light us more reddish. In the middle of the day the green light peaks. By avoiding this peak plants, so it was argued, are more efficient. Compare to a setup of solar cells. By pointing half of your cells more to the east and half more to the west a more constant power is generated, allowing a less powerful converter so a gain in efficiency.