If a red light is shone on a red rose, why will the temperature of the leaves increase more than the petals?

Answer 1

Because the leaves, being green, are reflecting a higher energy wavelength, than red. Ie, they're "darker", and therefore will absorb more of the light / heat than the red, which is the same wavelength. The red petals will reflect / absorb the same amount that is hitting them from the red light; the green leaves will absorb more. Therefore, higher temperature.

Answer 2

The red petals reflect the red light away keeping the petal cooler than green leaves that absorb the red light completely.

Thats what makes an object have color. You see the color of light that the object reflects. All other wavelenghts (colors) of light are absorbed by the object and go into heating the object up a little.

Further info about color: the sky is blue because the air reflects the blue part of the light that the sun gives off. The sun gives off white light containing all the colors but only the blue is reflected by the air. So the sky looks blue in color.