The net effect has to be measured at the level of the individual. Not the population. So, suppose that being red had a net BAD effect for the individual, but somehow a good effect for the population. After a few generations what red genes would be left to carry on this trait that has served the population so well? The nonred genes would have completely replaced the red genes. Evolution for the good of the population while to the detriment of the individual is called "group selection" which cannot work except in very limited circumstances (typically where individuals in a population are very closely related, such as honeybees, we call this "kin selection" in contrast to individual selection).
The point is that a given trait or mutation can have many effects, not just one. And it is the net effect of that trait that matters.
