What is this flower?
It's Verbena hybrida Voss (See here; the page is in French though). 



It belongs to verbena genus in the family Verbenaceae. It's about 10 to 50 centimeters tall with a squarish stem and opposite leaves.The flowers are small with five petals, and borne in dense spikes on the top. Verbena hybrida Voss is always used as ornamental plant. They are drought-sensitive, tolerating full to partial sun, and enjoy well-drained, average soils. It's always plant in warm and dry and sunny environment. About why a flower with two colors in one petal, it's because there are always two kinds of pigment in the petals. The pigments in different temperature and different acid alkaline environment present  different colors. When the flower is open, the acid-base property changed will lead to petals present different colors (K. Yoshida et al., 2009).