This might be a stupid question, but I am wondering if someone could fill me in on why some providers' stars are yellow in searches and others are not filled in with color? At first, I thought it was because there might be a min number of reviews necessary, like having 9 reviews or more, but then I saw yellow stars under one search where the provider only had 2 reviews.

Would be my guess.  Seems the stars go to yellow once the 5th review is posted.  

That was what I always thought but seems not to be the case reported in the OP. Sounds like maybe a problem report to TER -- if not a problem at least he might get clarification on when they go yellow and what we should make of that.

A quick scan of Minneapolis reviews seems to flip yellow on 5 reviews.

The amount of coloring in the stars is related to the providers scores.
Someone like Haley M has all five completely covered
As one moves down the list to an average score of about 7.75 only three are filled in
Just a quick way to compare.

Not at all sure about this but perhaps the one profile you saw that only had 2 reviews but still had yellow filled (or part filled) stars might be that some of the reviews she had were removed from the display. I don't think TER always deletes from its database, just blocks from display. Maybe those hidden review still count in terms of coloring the stars.

I think because my assumption was it would take 10 reviews to turn the stars from colorless to yellow when I saw that happen with 5 reviews, I just automatically read the 5 as a 2 - I am half blond - on my mother's side, lol. Anyway, yeah, I think Lester has it on the nose that at 5 reviews, they turn yellow. When I did another search under my usual cities, I couldn't find the one that I thought I had seen, with only two reviews. Once I started looking at the 5 reviews or higher, I saw the pattern clear as day. Thanks for clearing that up guys

Before 5 reviews none of the stars are yellow.

Which stars are you talking about?
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Search Reviews (using no special criteria or ANY criteria) and you get a list with 4 columns:
Name               Rating (stars)       Reviewed       City / Location
A....                     *****                 7 Reviews               City
                           9.1(........)
B....                     ****o                15 Reviews             City
                          7.2(7.5/6.9)
C...                      ooooo                3 Reviews               City
                               N/A
etc.
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If you HOVER your mouse over the stars, you see a pop-up box, e.g.,  
7.2 Hover --> Rating 7.19 (Looks 7.52 / Performance 6.86)  
7.52 + 6.86 = 14.38 divided by 2 = 7.19 rounds to 7.2
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I STARTED to think that the stars represent those averages in ranges:
(5-6)(6-7)(7-8)(8-9)(9-10) and that an average rating of 7.2 would color in the first two stars plus some of the third star that is hard to draw:
* * (third star) o o  
5 6 7.2 ..................
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Unfortunately, that 7.2 score had three stars plus part of the 4th star and an empty 5th star
* * * (4th) o
4 5 6 7.2   o
and it seems to vary with each Star Rating.  
Another 3 stars plus partial 4th star has an average rating of 8.3:
* * * (4th) o
5 6 7 8.3
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OR ARE YOU TALKING ABOUT INDIVIDUAL PROFILES?
Name  
TER ID: ######
Stars * * * * *  
.
Below that is a box with
Rating #
Looks # / Performance #
Chemistry STAR #
Location STAR #
.
And I still can't make sense of it.
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Is this one of TER's occasional contests? Like:
"The contributor of the 2 millionth (2,000,000) approved review gets free lifetime VIP!"
"Whoever can figure out how the Star Ratings work and explain it to us (TER Admin) gets free lifetime VIP!"

The number of stats filled or partially filled include the chemistry rating as well so the average of performance and looks is not all that goes into the formula.

Two years ago I've looked at how the formula works, broke it down right here:

Thanks for the link. I usually do some searching on my own but I didn't have time and I was late to the thread, as well. Good work. Let me guess: you scored an 800 on the Math SAT.  
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In the meantime, is there any explanation for the NUMBER OF STARS on the 'hit list' from a Review search? These stars:
Name               Rating (stars)       Reviewed       City / Location
A....                     *****                 7 Reviews               City
                           9.1(........)
B....                     ****o                15 Reviews             City
                          7.2(7.5/6.9)  
...  
The number of yellow stars + plus partially filled yellow stars does NOT seem to match the Rating numbers in a consistent way. There is no correlation to number of reviews, either.  
Mina / Minana    ****o 7.9 (8.3/7.5)      6 reviews Tustin Apartment
Sasha Valentine  ****o 8.3 (8.3/8.3) 104 reviews Chicago Hotel
Both have the 4th star filled but Mina is in the 7-8 range and Sasha is in the 8-9 range.  
Mina has a measly 6 reviews; Sasha has a whopping 104 reviews.
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There goes my "range" theory for stars:
(5-6)(6-7)(7-8)(8-9)(9-10) ****o works for Sasha and her 8.3 = 4th (8-9) star.
But Mina should be
(5-6)(6-7)(7-8)(8-9)(9-10) ***oo with only the 3rd (7-8) star filled, unless ...
(4-5)(5-6)(6-7)(7-8)(8-9) in which case ****o puts her in the 7-8 range.

Each star represents two points.

The Stars are the part you fill out that talks about chemistry.  You rate 1-10 on looks and service but give 1-5 stars for chemistry and something else. Those stars don't turn yellow in searches until the 5th review is published.

Please reread my link. I explain how the rating is calculated.