You will never ever ever find a good breeder breeding beige Newfs. Ever. If they are breeding a beige Newf, they have no regard for the breed standard and EVERY breeder should have great respect and determination to breed to that standard. If they don't, they are not even worth considering as a breeder. I know you appear to know this, but it also seems from your post that you are interested in the beige's. I say skip them... not worth the risk and I do not mean because of their colour... I mean because that breeder cares so little about the breed standard I would strongly suspect they give little consideration to other far more important aspects of breeding healthy Newfs as well. Very often these are the same breeders that don't think cardiologist clearances are a must-do, or they don't care about waiting until their dogs are two years of age and have all of their clearances... they usually go hand-in-hand. There are a number of excellent breeders breeding grey's though. They are just not as common.
Now, that being said, the difference between grey and beige is in the genes. A grey Newf occurs when both parents carry black and the dilute gene. A beige Newf occurs when both parents carry both brown and the dilute gene. A beige Newf is a double recessive because brown is recessive to black and dilute is recessive to undiluted. Since it is very simple to perform a DNA colour test, the double recessive beiges are very easy to avoid (and should be).
In terms of telling them apart, it should be fairly obvious if it is a grey or beige, but their eye colours are usually different. That photo you posted appears to be of a beige especially since the puppy next to it appears to be brown.