Thanks for the photo.......
...this effect is not, as I at first surmised, a result of the vacuum lift, but is part of the curing process and is a common 'feature' of some of the flags produced by smaller manufacturers. A common cause is stacking the flags in bundles before the curing has completed its initial stage, and so the flags on the front and back of the bundle are not as badly affected as those in the centre, as they 'dry' more evenly.
Basically, the concrete nearer the edges of the flags has dried out partially during the curing process, while that in the centre has remained 'damp'. The result is a difference in final colouration.
(It's not quite correct to think of concrete as drying. The curing process is a little more complex than that, and is a change in chemistry, rather than an evaporation of water content. However, the water content of a concrete
does have an effect on the actual curing process, and so, if the moisture level is not uniform throughout the concrete, you can end up with a colour or shade variation in the cured final product.)
Now, sometimes, this disappears over time, usually a couple of months, but I have known it to last for 12 months or so, but eventually, it [usually does disappear, although not always completely. In some cases, there is a faint but permanent difference in shade between the centre of the flag and the edges.
If you were to go back to the manufacturer, I'm fairly certain they would not be interested in an exchange or refund, and will probably give you the old line about them being a 'budget' or 'utilitarian' flag, rather than a top-of-the-range product, and, in the words of the prophet, you gets what you pays for.
In your case, I'd go back to the manufacturer/supplier. There's no harm in asking if they will offer a (partial) refund, as you're "terribly disappointed" with the finished effect. It's got to be worth a try. :)
I don't think youd get very far claiming it to be a manufacturing fault, although a good lawyer could argue that it probably does constitute poor manufacturing procedures. Your best line of attack is that the flags don't look the same as those in their brochure or on display in their yard. You need to go for the aesthetics rather than the composition.
Is that any help?