Here are the is the basics of the egg color.
If you cross a Pure White egg breed with a Pure Blue egg breed you get a pale blue egg. If you cross a pure White egg breed with a Dark Brown Egg breed you get a light brown egg. If you cross a Pure Blue Egg Breed with a dark brown egg breed you get an olive egg.
Are you in the UK? The oldest Cream Legbars in the USA were imported in 2010 so no one on this side of the pond has have them for 10 years yet. ;-)
In the UK there are tons of hybird blue egg lays, the Cotswold Legbar, The Skyline, the Fenton Blue, the Columbine, Sapphires, etc. The Cotswold Legbar was the first of the commercial Blue Egg Hybird and since Legbar is part of the name it has created some confusion to where many chicken enthusiast think than anything that lays a blue egg is a Legbar. These hybirds often are crossed with breeds that lay brown eggs. The first outcross is called the F1 generation. The 2nd generation after an outcross in the F2 generation. The F3 is the 3rd generation after an out cross. A line is not considered pure after an out cross until six generations are complete (F6) and all the offspring conform to the breed standard.
Some of the top show lines in the UK have been out crossed. I know of breeders in the UK that have experimented with out crossed to White Starts to increase production and Light Sussex to increase egg size. I am guessing that the Jill Reese line got stock from Emily DeGrey. I know that Emily's line got stock from a breeder that out crosses to Sussex so that could be the source of green eggs in the Reese line int he USA.
Unlike the UK Standard the USA standard specifies that egg colors for the Cream Legbars are Blue or green with no allowances for Olive eggs. Your eggs from your 10 year old line look great. The eggs from the hatchery should NOT be crossed with your ten year old line. They will ruin the good egg color you are getting.