If you have a look at the last 3 photographs on this link, there are two layouts that show the hills in use.
LINK – link
In my own experience, I think it is better to flock the hills in whatever mix you like or are using with the rest of your table and then to add large patches of a darker flock as this can lift the feature to the eye, enhancing the sense of high ground and also helps any photography that you may do.
In their own catalogue, Kallistra do this under their 'two tone' section, have a look because they give a photograph for each item they sell.
I like the hills because they have a decent height and the slopes are generally gentle enough for figures to be able to use them without slipping (you can add anti-slip material to the bottom pf bases.
I also did a short video (15 mins) about Kallsitra terrain which shows the various hill sizes.
LINK – link
Kallistra do two types of hill. You can get the pre-mould stuff as I show in the video, or you can stack tiles and then slope off the edges with their 'slope' pieces. This is good for be-spoke terrain such as doing the long Hastings ridge etc.