Read some David Benatar, Emil Cioran and Thomas Ligotti and you'll be able to write an essay about why we shouldn't have designer babies, or babies full-stop.
For designer babies specifically it should be obvious to most people that getting rid of genes which cause disorders and diseases is ethically upright and even unethical if steps can be taken to render the genes inert but aren't.
Cosmetic changes like hair color, eye color, etc. won't pose a problem for most people either. I don't know why it would, either way the child has no choice what color its hair will be, but it will fit the personal preference of the parents. It could even be argued that it would be unethical to choose ginger hair, but I doubt that's a point you'd want to make.
The big issue for me is when they begin to edit the genes which govern intelligence. Some articles are saying that this isn't possible at the moment, others say it is, others say it eventually will be. So I'm going to assume that if it can't be done yet it will be able to be done eventually.
There's a book called The Bell Curve which looks at General Intelligence between different groups of people and the main message behind the book is that eventually there will be a split between the rich, intelligent elites and the poor, dim witted plebeians and the elite will be the minority because in the working marketplace there has been a radical increase in the value of highly intelligent individuals because jobs have generally become more focused on mental ability as opposed to physical strength. The highest paying jobs aren't builders or factory workers, they are CEO's and software engineers.
The worry is that Designer Babies with intelligence edits will add to this eventual outcome. Considering that it will be the rich who are able to pay for this designer baby treatment and they will be the ones passing on their wealth to these children, it contributes to this trend of the upper class becoming more intelligent and wealthy than the lower class and this divide will only continue to increase.