Color coding in the legend is for the age of the bedrock under all the loose sediments and soil. Yellow and peach are the youngest rocks, followed by green Mesozoic rocks, and the older Paleozoic rocks. All these rocks are sedimentary: limestone, shale, sandstone claystone, and coal.
The far eastern part of the state is more mountainous, and is underlain by mostly igneous and metamorphic rocks, shown in bright red and burgundy. These rocks are resistant to erosion, so stand high over time. Just to the west is a band of bright pink rocks. This is the Valley and Ridge province. Some layers are resistant and form long ridges. Other rock layers are soft, and weather to form long, somewhat narrow valleys.
More information is at this webpage by Clay Harris.