Here are some of the violets I have come across on my walks.
There are over 500 species of violets, and there are many cultivars, so I may have misidentified some of them.
This tiny one is a field pansy (Viola arvensis), akkerviooltje in Dutch. The flower is only 1cm (0.4") high:
The rare marsh violet (Viola palustris), moerasviooltje in Dutch, as seen in the local peat bog:
Wood violets (Viola riviniana), bosviooltjes in Dutch, seen in forests:
This is a "heartsease" (Viola tricolor), driekleurig viooltje in Dutch, from which many of the cultivated pansies are derived:
And finally, an unidentified species, no idea what this is:
Thanks for watching!