i've seen the smaller wild all yellow and white versions so it is quite possible those could cross with any of the domesticated ones and give all solid color results.
If they're REALLY tiny (like pencil eraser sized*), that's V. arvensis, field pansy.
Solid color for any of them is not all that uncommon. Pretty much anything you can find in a big pansy, you can find in a small viola (though, as I have mentioned, bright red is largely missing from the violas).
I did have ONE case many many years ago of a tiny volunteer plant that made a flower the size of a field but with a domestic color palette (yellow with a black center). But it didn't make any seed.
I also had ONE case of an al yellow viola that developed purple polka dots on the flowers, but those seemed to fade out as the plant kept producing.
the birds or other animals must spread violets (and violet like) seeds around. we've had them show up and to start with there weren't any around at all.
Ants, actually. When they come out, violet family seeds have a little fleshy aril (it's that tiny white thing you see if you open a fresh pod) The ants collect the seeds, carry them to their nests, eat the aril and then dispose of the seeds. That's how they do their far spreading (their near spreading is done via the pods exploding, or why it is best to collect the seed early in the morning or on a cloudy rainy day before they dry out totally.
did they come in via wildflower seed blends that were put around? hmm. good question i cannot answer for sure, but maybe?
I've never heard of anyone adding violets to wildflower mixes. For one thing, most violets like shade, and wildflower mixes tend to be for full sun.
note i do not recommend using those blends because they may also include random weed seeds that can become invasive, plus, well the plants chosen to include in that mix may also be ones you don't really want.
Exactly. Don't use a mix, buy individual types and make your own. Some of those things are perennial, which is great if you are trying to make a permanent patch (or return a chunk of land to the natural cover) but not great if you want to be able to alter what is there in the future. Blue Lupine and Coreopsis seem to be the biggest offenders, both can take over over time (oh and Rudbeckia, Black Eyed Susan).
And yes, there can be weeds seeds. No offence to their goals, but I have noted that Prairie Moon is particularly bad about getting other things in with what you are buying, particularly what seems to be some form of ragweed.
V. arvensis CERTAINLY can get into mixes, though usually lawn ones, since it's weedy. I think it also likes to inhabit tree nurseries, since most of the examples I have seen were either poking out of the burlap of wrapped up trees being sold or around places where bushes had been planted in parking lots.
* Though I did see one this previous spring with double sized flowers, and the plants in the "parking lot" patch seem to be bigger both as plants and flowers than the "burlap" ones.