I hv a hp photosmart 7200 series printer and the manual says I need to use a HP#57 tri-color and HP#58 photo print catridge.
My question is whether I could use the HP#56 Black for both slots instead.

Thanks
KK

No you can't. You have two cartridge wells. The left hand well takes the #57. The right hand well takes either the #56 or #58. Using the #58 gives you better photo printing.