I think I have a couple reasons why I love Berry Sweet Sims so much. First off, I am also one of those people that is IN LOVE with bright colors and things of that nature. I mean, my favorite color's blue, but there are so many other colors I love as well. Sometimes I can never choose.
Second, I think the variety of colors makes certain features stand out more. Like for instance, if you had a regular vanilla sim with blue eyes, while that would look nice, it wouldn't be something worth remembering for long. Then you take a Plum-colored sim, and then have those same blue eyes, it makes them stand out so much more, and the colors compliment each other so well that it just works out perfectly.
That was just one example, but that's the gist of it. Also, I think the color also exemplifies their personalities and emotions better as well. Maybe it's the boldness of the colors, or lack there of, but it just works.
Now, don't get me wrong...vanilla sims are awesome when I need them (with my neverending ideas for machinima that I could never do at the frequency I think of them...I swear I wish I'd have someone collab with me for said ideas LOL), but I only play with Berry sims now. I've been spoiled.