They're both so close that's it's not really possible to tell on a computer screen, but I'll put the images below. Our Authorized Dealers should all have fabric swatch books at their stores. Go to bestop.com where-to-buy to find one near you.
Black diamond has little diamond points in it - it's really small, very subtle, but it catches the light a bit more and looks shinier. Black denim is exactly what you think it is. In my opinion, they are close enough that 97.3% of the people will see your top and just think "black." Which is a good look on the Yellow!
This is diamond. See the little "points"? Yeah, me neither.
This is denim. Fabric pattern like jeans: