I want to prepare serum from blood but I did the following:
I have centrifuged immediately without leaving the sample to stand from 20-30 min to clot. Should I discard these samples or can I use them noting that the samples were yellow in color after centrifugation with a red pellet.

Thank you for your help.

Well, if no/little coagulation occurred, then what you have is primarily plasma instead of serum. If you will be analyzing the material for typical clinical chemistry parameters (liver enzymes, glucose, indicators of renal function, etc.), then this probably doesn't make a difference.

Good luck,
Sam