First of all, in order to do genetical engineering of corals or whatever organism for whatever traits you want to modify, you NEED to know the genes ruling those traits. The more complex the character, the more complex its genetic determination (that is what we call polygenic traits). Therefore, a lot of genetic, genomic and proteomic analyses have to be carried out to have an idea of the genes governing the trait you want to modify. That is why those traits determined by a single gene (some diseases, the color of some part of the body, the resistance to a certain disease, etc.) are more easily manipulated than complex traits like behaviour.
There is a lot of ongoing work in plants to find genes whose products make plants more resistant to water stress or extreme temperatures. I'm not so sure about work aiming in the same direction in corals. We already know some gene variants that affect color but, as Randy cleverly pointed out, what is the point, beyond fun or simple profit, to genetically modify a coral to change its color?
I agree that finding the genes, or the alleles, that could make corals more resistant to lower pH or higher temperature, or even trying equivalent genes in plants, would be very positive in terms of fighting the negative effects of global warming.
