The most common colors of bell peppers are green, yellow, orange and red. More rarely, brown, white, lavender, and dark purple peppers can be seen, depending on the variety. Most typically, unripe fruits are green or, less commonly, pale yellow or purple. Red bell peppers are simply ripened green peppers, although the Permagreen variety maintains its green color even when fully ripe. As such, mixed colored peppers also exist during parts of the ripening process. Green peppers are less sweet and slightly more bitter than yellow or orange peppers, with red bell peppers being the sweetest. The taste of ripe peppers can also vary with growing conditions and post-harvest storage treatment; the sweetest fruits are allowed to ripen fully on the plant in full sunshine, while fruit harvested green and after-ripened in storage is less sweet.

are green bell peppers just unripe red bell peppers?
yes