Looks like Rachel has wet feet - the soil looks very wet.
The base may be corking which is not a disease. But see if the base is firm; if not - it could be slowly rotting (again, looks very wet).
Not sure what you refer to as sticker parts but I am assuming that you mean areoles with spines (the prickly things/thorns) - nothing wrong with the color.
The pink part is grafted on the green one because pink can't survive on it's own (no chlorophyll).
The plant should be in well draining potting mix, in a pot with drainage hole and in sufficient light.
Rina

Yes, too much water, probably not enough light. These in my view are only temporary plants, one of the first a novice might buy. If the browning on the base is corking, as Rina suggests, it is not a problem. But if you touch that area, and it is soft, it is root rot, and you need to cut above it, to just green growth. Let it sit out for a week or so, for the cut to heal, then repot into a dry, well draining medium, and do not water until you see active top growth. As I alluded to earlier, these generally do not live too long. Either the rootstock rots, or the graft fails, and you get rot on the top.

Christopher

I had just watered her before I took the picture. If that changes anything, and it feels fine not too soft. rina_ and kaktuskris

It is a cleft graft.

Actually it is a flat graft for these, they just trim the edges for a better seal.

Christopher