I thought pure Yellow shirts are against the rule. I've found Killerspin makes yellow shirts and their sponsored players wear it.

I would say that if you are playing with a white ball it wouldn't really matter but from a Umpire perspective I would say follow the ITTF and USATT rules on the shirt colors and they ask for dark colors.

I have seen the Chinese players like wang hao wear yellow shirts. The brand DHS makes for them. I think some people will have trouble seeing the orange ball on a yellow background.

My say is that follow the ITTF and the respective national body like USATT rules on the shirt colors.

Chinese players like Wang Hao who wear DHS yellow shirts - this could be sponsors but in the tournament where Wang Hao plays, what was the national body that sanction wearing yellow?

Yes I agreed with varghese's comment on people having trouble seeing the orange ball on a yellow background; it is better to wear dark ccolour apparel when using a white ball or orange ball.

I have noticed Jorgen Persson was wearing a yellow shirt at the Olympics when he has played against Wang Liqin at the Quarter Final.

Well it seems that they were allowed,
After all they are champions,
But maybe their should be a rule?

Some referees are strict and some are not. Jorgen's opponent can complain about the color, and the referee can take action.

I think as long as the color fo the ball does not match with the color of shirt, it is okay to wear the shirt. You can see an orange ball on a yellow background is not a problem. To me, it may not be a problem. In a low light condition that would be a problem to me.

Yes, Because it would be unfair.
But what would happen if he did complain?
Would Jorgen lose automatically?