Need help in choice cable for my PSU?

Solution
Yes, both CP-8920049 (red) and CP-8920050 (white) are compatible with the RM550 Corsair PSU:

AXI Series – 1500/1200/860/760
AX Series – 860/760
HX Series – 1050/850/750/650
HXi Series – 1200/1000/850/750
TXM Series – 850/750/650/550
CXM Series – 750/600/500/430
CSM Series – 850/750/650/550/450
RM Series – 1000/850/750/650/550/450


Note: * ATX 24PIN CABLE SOLD SEPARATELY *

Source:
http://www.corsair.com/en-us/professional-individually-sleeved-dc-cable-kit-type-3-generation-2-red
http://www.corsair.com/en-us/professional-individually-sleeved-dc-cable-kit-type-3-generation-2-white
Yes, both CP-8920049 (red) and CP-8920050 (white) are compatible with the RM550 Corsair PSU:

AXI Series – 1500/1200/860/760
AX Series – 860/760
HX Series – 1050/850/750/650
HXi Series – 1200/1000/850/750
TXM Series – 850/750/650/550
CXM Series – 750/600/500/430
CSM Series – 850/750/650/550/450
RM Series – 1000/850/750/650/550/450


Note: * ATX 24PIN CABLE SOLD SEPARATELY *

Source:
http://www.corsair.com/en-us/professional-individually-sleeved-dc-cable-kit-type-3-generation-2-red
http://www.corsair.com/en-us/professional-individually-sleeved-dc-cable-kit-type-3-generation-2-white
 
Solution
It's an optional item, IF you don't have the ATX 24-pin in your RM550 PSU. The only reasons you are buying those individually-sleeved cable kits are 1) your PSU's cables are missing or not working, and 2) for aesthetics.

So, if you want to match all cables *or* your PSU's ATX 24-pin cable is not functioning, then, you can get the sleeved 24-pin cable set alongside the other cable kit.