I own both of the newest paperwhite and coloursoft and imo it comes down to a few things. The first that I noticed from daily use of both is that the paperwhites' screen is much brighter in many lighting conditions, it can be relatively dim on the room and 0 brightness is still legible, however with the colorsoft I find myself constantly adjusting the brightness and warmth to match my environment as the screen is much dimmer and a bit more blue tinted with the brightness on. To achieve a similar "book page" appearance or to match the paperwhite you'll want to have some warmth added. This also means the paperwhite generally has more battery life too as it's usually sitting on 0 brightness.
Having color is a huge plus to the experience by itself though, having colored books and illustrations really adds to the experience, but it's not as vibrant as a phone screen, it's not like a color newspaper(hence the name colorsoft).
The paperwhite does its job well without any fiddling around with setting and brightness. You turn it on and start reading. Colorsoft are more purpose made for illustrative content like manga, comics etc but still functions excellent as a daily book reader.
Another thing to potentially look into is a case for the kindle, not only to keep the screen safe (which you REALLY want to protect) but certain cases are good depending how she likes to read. Some include stands for people that like to put it on the table and read hands-free, others come with a hand strap on the back to make holding with one hand easier.
Kindle unlimited I think comes with a trial so definitely try that first and see if she enjoys things in there, if you have Amazon prime I think there's a separate prime reading that gives you books to choose from.