Sonus clay is kinda white...definitely not yellow. I bought the clay & lube as part of a kit a few months ago from Autopia and really liked it. I had used the Mother's clay about 6 weeks earlier and even though my car is new all of the surface contaminents were not removed - probably more my technique than the Mother's clay.
I really liked the Sonus clay. I spend a long time doing my car and got it almost perfect. The clay was very easy to work with up to a point which may have resulted from using too much lube. I split the bar into thirds and tried to use one piece for the entire car. I probably should have used half the bar and split that in half too. When I was about 80% done the clay became pretty sticky in my hands...it was still okay on the car, but I could no longer knead it without significant chunks coming apart and sticking to my hands. I broke off a small piece and finished the car without incident.
Even with the problem I had, which was probably self-induced, I really liked this product. My car had a dealer applied sealant that I was able to remove completely with the Sonus clay...the Mother's took some of it off, but not all...again that could be my technique but I found the Sonus to be more effective.
There are some reviews on Autopia that you should check out before buying. I believe all were positive.