Doesn't Involve Legacy, Custom Content or Glitches:

I like to have my Sims own Stores.

I there any Computerized Rhyme or Reason why sims Buy one thing over another?

Paintings, chairs, endtables, lamps, appliances, stereos, guitars, cars?
Color, size, price, location, fun, utility?

Any Clues or Guesses?

It depends on the sims. If they are your [possibly poor] playables, they can only buy what they can afford.

If it's townies, they have infinite money so they are likely to go for whatever you suggest.
Tip: you can click on an item on sale and show it to a customer. Townies will almost always buy it if you dazzle them [gold sales badge].

What's a bit silly is sims tend to buy multiples of the same items if they stay on the lot for longer, they will buy, say, 4 copies of the same thing.
I believe there is a mod which makes them go home after purchase instead. I think it's this one https://simfileshare.net/folder/87840/ SW-CustomersLeaveAfterPurchase

What definitely SHOULDN'T affect it is recolour.

They of course have no concept of "need". Like if they don't have a TV at home, they won't be anymore likely to buy one. They seem to just go for items priced lower relative to the standard price. Maybe also smaller items if the prices are at the same level. With bigger items the buy bar might not have enough time to fill. You can earn the maximum wholesale discount and set expensive TVs and such at below normal price. Since Dazzle costs energy, you would do it on those big TVs only. I made an experimental mod for myself to make them choose this automatically, but didn't finish it.

If visitors have no family funds because what they buy actually deducts, unlike when they go to buy coffee or restaurant, they will come shopping but show a crossed over money symbol, buy nothing but give you some minus rating.