Ri, I assume you've already finished the project you were working on but thought I'd let you know one of my favorite things about Koh-i-noor pencils when it comes to shading... they are erasable! So, if you want the center of your image to be REALLY light, you can actually take an eraser (I like to use a white eraser) and erase a lot of the color from these pencils.
Now, it doesn't work as well with the Prisma colors, but you can erase some colors from them as well.
Another tip would be to build your color as Theresa mentioned - but leave the center of your image completely white. Blend the dark colors inward gently being sure to leave the center until the very last. You can always add more color to the edges. I've been experimenting by adding a brown or even black pencil line in a few of my very outer edges and then blending it out to add even more shadows.
I'd love to see your project you mention in your reply - have you shared it with us on STV yet?