But I am drawn to certain colors (and NOT drawn to other colors); and if I need a themed kit, then I buy one. Or five. Example: we're going to Legoland and disney in a few months, so I bought some Project Mouse goodies cuz they'll work for the lego pics as well as their intended disney pics. I want that album to be cohesive (normally I don't care about that), so buying a few packs really made sense for me. 
I wish I had some type of pattern to my buying. I just buy what I like when I like.
And then there's BYOC: if I love the palette (which is 99% of the time lol), then I buy a lot. Take this month, for example. I loved the palette and I loved the themes from basically every designer. It didn't hurt that the day it came out, DS went to a bday party at an amusement park-type place, so buying Kristin Aagard's stuff just made sense in order to scrap those pics.
Nowadays I mainly go by designer and colour. Some designers just create exactly what I like, but I don't buy everything they release. The color scheme must be right for me. And the kind of elements. Too many flowers and such is a no for me, because I don't use them much.
I hardly ever buy themed kits or, more to the point, the theme is not the main reason I buy. I use themed kits for everything, just like @jk703 said.
I used to buy more project based but it always turned out that I changed my mind and used something else and the specifically bought products were tossed aside. So, now I go simply by whatever catches my eye colour wise. .
The only exception for a long time was a farm kit by Kirstin Aaagrad I bought to scrap a page about Stardew Valley, and guess what! That page still hasn't been done,
