That's because they are different varieties. Colored bell peppers are cultivars specially bred to turn those colors as they near full maturity, so they are different from regular green ones that way. But most all bell peppers are the same in that they all start out growing green. It's like colored tomatoes - they all start out green & change as they get bigger. They may all look the same when you buy them & as they first start putting on tomatoes, but as they ripen you'll see the difference. So if you buy a regular green bell pepper cultivar, it won't turn all-yellow because it wasn't bred for it. And it may eventually turn red, but it likely won't be that gorgeous all-over red color like red cultivars bred for it.

Not sure if people have always been confused about peppers, that people don't understand that greenish (can be yellow green to dark green) are unripe, then they ripen to red (or yellow, orange). Many people think green bells and red bells are different types of plants. Purple color will ripen to red. Some types go from greenish to purple to red. Not sure why thick walled blocky bell peppers are so common in the US, but they are what people buy. Bell peppers are lousy at ripening in the humid hot Midwest Summer. The bells sit there and then when they start to ripen they rot ruining the pepper. Longer or less thick peppers can ripen easier. I find peppers ripen better in mild weather, sunny warm days with cooler nights.

In zone 8 you have plenty of time left for theose peppers to ripen. I've always thinned the peppers a little by picking a few of the full sized green fruits. The remaining peppers will ripen sooner. Keep in mind that you'll have less ripe peppers but usually more usable peppers, less spoiled ones. Not sure which varieties you are growing but there are a number of varieties (Early Snsation, Zsa Zsa) that begin yellow and are fairly sweet as unripe peppers go. You might want to try just a few of those next year to complement your large yellow bells. I'm not going to pick on Bill, but the yellow bell pictured would not be marketable for me. I mention this because it is common to have overripesides, sunscald , bug damage and especially rotting on peppers as they are left on the plant longer. I pick 90% of my bell peppers with some amount of green on the fruit and my customers accept that. There are even a few (one emailed me for a few bu. of 50%yellow just a few min. ago) that prefer some green to assure themselves that they will have firmer peppers for processing.

harold59 same thing happened with my peppers only with jalapeno and serrano and the same hot. I have picked a short fat jalapeno and a longer bullet shaped serrano off the same branch. Some of the peppers get about the size of a nickel and turn red, they have a sweet tast when first bitten into then feel like a match head for about 20 minutes. Guy at works claims to eat Ghost peppers but admits my peppers make him sweat. I have a cayenne plant also they are a little hot but real bitter. I planted 3 Jalapenos directly in the ground but the serrano I had bought earlier and kept in a pot in the house when it got a pepper I put it outside close to the jalapenos and brought back inside at night after the jalapenos started blooming I planted the serrano about 5 feet away. the serrano only has serrano looking peppers on it, but they are dark green before turning red. in the spring i'm going to add a bell pepper.