Of course we do not know exacty but there are multiple levels of evidence now that chimerism is very rare. One example would be cancer sequencing were you compare DNA from the cancer to a control tissue. I haven't heard of any of those mentioning any cases. The other case would be the recent GTEx concortia that sequenced upto 40 tissues of 600+ humans and did not detect any obvious chimera cases.

Different colored eyes one the other hand my be caused by a lot of things like no-genetic reasons, but also somatic (i.e post egg fertilization) mutations like loss of one chromosome copy and others.