Solotica Natural Colours have a slight limbal ring on the outside of the lens and have black accents / pixels across the colourized area which make them look slightly darker from the distance.
Solotica Hidrocharme are now discontinued. They had a strongly defined limbal ring and were suitable to create a more dramatic look.
The colour of the monthly Solflex and the yearly Natural Colors is very similar.
The pattern of colour distribution on the lens is slightly different. Furthermore, the limbal ring is more defined than on the yearly Natural Colors - more similar to the Natural Colors