Want to see what I give to yellow?,
do not leave it up to me,
for all you'll receive is black,
so use me to acquire white.

What am I?
Line 1 - I think that "yellow" is a person, but the only person I can associate the colour yellow with is Cloud.
Line 2 - This line is saying that whoever the answer is, they are untrustworthy.
Line 3 - Whoever it is will end up giving/using/whatever the Black Materia.
Line 4 - "Use" them to obtain the White Materia.
Line 5 - What has been used in place of Who,and I think for a good reason. The answer is not a specific person or creature imo.

With that, my answer is JENOVA Cells. They give yellow (Cloud) extra strength. But the cells take control of Cloud and he takes the Black Materia and gives it to Sephiroth, so they aren't too reliable. Not sure how it relates to the White Materia, but worth a shot.