Why do the roots of plants usually look white, rather than green?
The direct answer is that the root does not contain chlorophyll.
As we know, the leaves are receiving sunlight and converting light into starch, and the reason why most leaves look green is that they have chlorophyll.
Chlorophyll can absorb sunlight and convert it into starch. And the sunlight is composed of lights of different colors(light color is determined by there frequencies), chlorophyll cannot absorb all the sunlights, one frequency it cannot absorb is green frequency. These lights are reflected into your eyes, so most leaves look like green.
Because the roots are underground and they don't receive light, so there is no need for them to have chlorophyll. And that's why they are white and leaves are green.