You have been had I'm afraid. Welsummers (so named as the originated in the village of Welsum in the Netherlands) are layers of dark brown eggs.
Yours must of had a blue egg layer bred into it somewhere along the lines, my guess is it's been crossed with crested cream legbar.
If a bird has both the brown and blue egg laying genes the eggs come out green.
Pure bred welsumemrs lay dark brown eggs it's part and parcel of the breed. Take them back and get your money back and tell the breeder they need to look up the breed standard before trying to con folk!
A pic below, bad light but you get the picture. The brown ones are what a welsummer egg should look like:)
