We have no way of knowing which is the most accurate. The color of the scale has zero do to with accuracy.
The most accurate scale is the type like they use at the doctor's office that have the weights at the top that the nurse slides left or right until the bar with the weight indicators is totally horizontal. Some supermarkets/stores also have those that you can use (sometimes for a small fee).
Your results for 2 of your scales are close, so either both of them are off by several pounds, or the one with the 171 lbs is. Odds are the two with the lower weight are closer to accurate, but that's just an 'odds/statistics' answer. Not knowing if any of them are digital, if they are calibrated properly, and if you used them on the same surface in the exact same spot (move 1 scale, put another in its spot and weight, then move that one and put the 3rd in the same spot and weigh), it's impossible to give a good guess.
It doesn't really matter because if you're happy with your weight, the number is immaterial. If you aren't happy with your weight, then as long as you make changes that causes the number of any of those scales to decrease, you're losing weight so headed in the right direction.