Changing the color of a group of item is only possible when the 'color property' of all the pieces are the same, that's what I know.
For example, if you made a group of items entirely out of gutter pieces(gutter cap, gutter 0.5m~2m, pipe 0.5~2m), whose color property is 'body paint color', you will be able to change the whole group's color. What I usually do is split the group to smaller parts with the same color properties to make them easier for me to change colors.
Though I don't think we can change the blueprint's color before we place it, like fountain effects. I too wish I could change the color of multi-object group at once like we can change the color of door/window/icon of staff facilities.