There was a range of purple-violets, amongst which there might have been some 'pure bred' Lucerne plants. But here are some pictures of the more extreme colour forms.
Presumed Sickle Medick, though no seed pods for a positive ID at this stage
Stunning deep red-purple flowered hybrid - looked near-black from a few feet away. There were three of these fairly close together.
Both the above were at the southern end of the marsh by the Cray, before the big spoil tips.
White-flowered hybrid with a touch of purple-violet in the more terminal flowers.
Lovely green-flowered hybrid with the more terminal flowers in purple-violet.
These latter two, along with a couple of pale yellow flowered specimens, were just north of the flood control gate on the River Darent, near its confluence with the Thames.
I don't know anything about the genetics of flower colour in these plants, but flower colour can be controlled by a multiplicity of genes, so I suppose it's conceivable that this range of flower colour could arise in first generation hybrids between the two species, especially if the genes are not completely dominant, as would appear to be the case with contrasting colours in the same flower cluster. But it seems more likely that the hybrids are themselves fertile and capable of interbreeding with each other and both parents. I'd hazard a guess and say that might be the route to the green flowered specimens.
At any rate, to my mind these would make very nice late-flowering ornamental plants for garden use, though one might need to grow a number of seedlings and retain the best colours.