While pearls can naturally occur in every hue and be very saturated in colour, pearl colours can be treated to alter their appearance. The most common colour treatment is dyeing, and it is more common with freshwater pearls than saltwater pearls.
While freshwater pearls do come in a wide range of beautiful natural colours and can be quite saturated, they typically are not naturally dark in tone and do not naturally occur outside of the warm purplish, pink, orange and yellowish hues. So if you come across freshwater pearls that are grey or black, very vibrant purple or pink, deep red or orange, or any green or blue colours, you should enquire as to whether the colour is natural or treated.
It is a good idea to enquire whether colour is natural or treated when shopping for any pearls, and to always ask for any declarations in writing upon purchase.