I can't see any green ones, just plain dark red and brighter variegated ones. If its the bright ones you want cut the others off. Cut off all the ones you don't want right back to where they are growing from..
But this picture isn't spirea japonica its the other one you asked about.

Yep, the Coprosma. If you want to keep the bright red variegated leaves, just follow the branches that have those dark burgundy/green leaves down to where they come out of the rest of the bush, then cut them as close to the main branches as you can. If you do it now, it won't leave much of a hole in the bush. If you wait until the green branches cover more of the bush--likely, because they grow faster than the variegated ones--you may have a scrawny mess to rehabilitate, later.