The Reach is the blooming garden of Westeros. Trade thrives from port cities to its east and southeast, as well as along the Roseroad that connects it to the west and north. Here it is common to see people from every region save the North and Vale, however diversity flourishes through its closer connections with the Summer Isles and other connected international trade.
The Tyrells are the current Lords Paramount of the Mander and Reach after the extinction of their ancestral House Gardener during Aegon's Conquest. House Gardener in fact had many branches through marriages to other houses, some of which still hold powerful seats in the breadbasket kingdom. The current heirs to Highgarden are connected also the Redwyne, through their paternal grandmother, and Hightower through their mother.
House Hightower claims to have First Men ancestry, although some believe them to have ties to Valyria due to the frequency of their silver hair when not golden. (Perhaps this trait reappears through the usage of magic within the line? More research to be determined.) Heirs often have green or dark brown eyes. The seat of their power, Oldtown, is the oldest city in Westeros, and the presence of many different peoples can be seen in its population. The majority claim Reach, Westerlands or Ironborn descent. There are far more foreigners from southern and eastern lands to be found than those that claim descent from the Westerosi North.
House Redwyne is of the Arbor, which not only has history under Ironborn rule, but also much trade with the Summer Isles and beyond thanks to its notable Arbor wine sales. Though they have an old rivalry with Dorne, the result of both battles and trade between them means there is a sizable population with Dornish roots. This leads to the ability to deeply tan before burning in the hot sun of Summer for those of House Redwyne. Redwyne also controls the largest fleet in Westeros, and this draws many seafarers of all backgrounds to its lands.
Willas Tyrell is the heir to House Tyrell, and his dark brown curls matched with green eyes have people whispering of the resurgence of House Gardener traits. It is part of why many believe Mace Tyrell did not name Garlan his heir after Willas' accident that broke his leg. (In reality, it is more that he learned how to rule from his grandmother.)
Garlan is the second son with has golden brown hair and green eyes. The golden tint is known to come from his Hightower ancestry. His skin is a light tan due to the large amount of time he spends training outdoors, while Willas is the palest of the siblings due to his injury limiting his excursions outside of Highgarden's walls.
Margaery and Loras are the youngest. Although not twins, even Cersei Lannister admits them to be near identical. Both have wavy caramel brown hair and hazel eyes, along with soft jawlines and a slight build. Loras, while younger by a year, is slightly tanner than Margaery due to his activity training as a knight.
Much of the Reach, especially the lands around the Mander, are lush farmlands. As such, many of the commoners work in agriculture, and the large amount of sunshine coupled with labor in the fields leads to a darker population than the rest of Westeros, save Dorne. However, this tends to not apply to those of the noble houses due to them spending less time outdoors and taking steps to protect their skin from the sun's harm by using silks, parasols, and hats.