You've been told that having a certificate or a diamond grading report is important, & as a responsible consumer, you get one - unfortunately, you probably won't understand a word of what is on that diamond grading report, unless you are a jeweler.
On the color grading scale, D, E, and F mean that the diamond has no color. G, H, and I means that it has very little color. J, K, and L means that the diamond has a slight yellow color. P, Q, R, S, T, U, V, W, and X means that the diamond is a darker shade of yellow. Z means that the diamond has a fancy color - other than white or yellow. On the color grading scale, D is the most valuable, and X is the least valuable - however diamonds that get a Z rating are the rarest and most expensive diamonds in the world.
There are many aspects to a grading report. Figuring it all out can be very confusing. You should talk to a jeweler you trust, and have them explain everything on the diamond grading report to you.