The reason why roses are red while some petunias are blue is due to the different pigments present in their petals. The red color of roses is primarily caused by the presence of a pigment called anthocyanin. Anthocyanins are natural pigments found in many plants and are responsible for a wide range of colors, including red, purple, and blue. In the case of roses, the specific type of anthocyanin responsible for the red color is called pelargonidin.

On the other hand, the blue color in some petunias is the result of a different pigment called delphinidin. Delphinidin is also an anthocyanin, but it differs in its molecular structure from pelargonidin, resulting in a different color. The presence of delphinidin is what gives certain petunia varieties their distinct blue hue.

It's worth noting that the colors of flowers, including roses and petunias, can be influenced by a variety of factors such as genetics, environmental conditions, and even the soil in which they are grown. As a result, there may be variations in the shades and intensities of the colors observed in different varieties of roses and petunias.