It looks good to me. The yellow probably is bruising or stain from the antiseptic that they used. An infected incision will be red, swollen and 'angry' looking. An infected incision will often be draining as well.
Thank you. I've been so worried because I've never had a cat or any animal for that matter spayed. And I've been freaking out over it. On the forth day so I'm hoping I'm doing a good job.