If it's a matter of changing the color of them to another, then add the new color to your swatches and then delete the old color (it'll ask for a replacement swatch to change the old color to a new color, i.e. the new swatch you just added.)

As for a group selection, I don't believe you can select an object based on color.

The find and replace feature allows you to find objects with a fill or stroke of a particular colour (or half a dozen other criteria).

Tim