Reason behind colour of hibiscus and sunflower
Flower color is result of reflection of light due to various color pigments. Evolutionary such pigments were selected due to various reasons like to attract pollinators (Dyer et al 2012). There are two major types of flowers pigments 


Carotenoids (produces yellow, orange and red colors)
Flavonoids (produces red, purple, magenta and blue colors)


However, flower color is affected by various other factors (e.g. other pigments, metal ions and its chemistry etc.) along with above pigments. 

Hibiscus' dark red color is outcome of Anthocyanins which is water soluble flavonid (Grajeda-Iglesias C et al 2016). However, this pigment is highly unstable and depending on pH of the surrounding it can be red, purple, blue or black. (More details in this book). Hibiscus contains majorly delphinidin-3-O-sambubioside (Dp-samb) and cyanidin-3-O-sambubioside (Cy-samb) as the major anthocyanins (DU and Francis 1973). Chemical reaction responsible for generation and utilisation of these pigment is shown below (borrowed from Grajeda-Iglesias C et al 2016)



In above image, at low pH values $AH^+$ produces red color while  and $(A)$ produces purple/blue, which is thermodynamically unstable. In addition to that co-pigments like Hydroxylatedbenzoic and cinnamic acids, hydroxyflavones (which all are colorless) are also responsible for not only stabilizing these chemical products but enhances the color (Grajeda-Iglesias C et al 2016).