So! In Black & White, there's a bit of wordplay going on with the numbers of which Kuro and Shiro are given. I'll put the relevant caps and explanation under the cut for reference.
So these two share the same number! Each Angel and Demon is given a number when created and through happenstance, these two have the same. And they're also a demon born angel and an angel born demon!
Coincidence! Or fate?
. . . and then the demons and angels break out into war. True story.
And there you have it, the sad tale of Kuro and Shiro.