What will happen to the concrete I tint with latex paint?
Latex paint added for color, strength or work-ability will not harm concrete, it actually has a chance of improving it.
The scientific article from New Zealand with link someone posted here actually states that  optimum strength is achieved at 12% to 16% paint replacing water.
http://www.hms.civil.uminho.pt/ibmac/2008/14IBMAC_123.pdf
As far as the original post information, I did not read anywhere in the article that a  60% paint ratio to 40% water ratio was suggested. Article states that too much paint is not good, nor does too little accomplish much.  Other online research would lead me to the conclusion that I should not exceed 20%.
One site suggests that amount plus 10% to make a wetter mixture, but I trust the scientific study info more. An excessive amount is a bad idea, but at the 12 to 16%  levels the concrete is actually improved over a water only mix.
I am using paint for purposes of adding color, and getting rid of old paint, but will follow NZ article guidelines for strength and work-ability.