That orange color comes from the presence of carotenoids in the flowers and in the pumpkins. The Cempazuchitl is full of lutein molecules, instead the pumpkins contains beta-carotene molecules.
Ese color anaranjado lo originan la presencia de carotenoides tanto en las flores como en las calabazas. El Cempazuchitl contiene muchas moléculas de luteína, mientras que las calabazas contienen moléculas de beta-caroteno.
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| Lutein molecule |
The structure of these molecules is very similar. In particular, both of them contain a double conjugated bond system, responsible of the orange color. The presence of a double conjugated bond system in the molecules implies that the molecules are almost planar due to sigma bonds along the system, and it also implies that the molecules contain delocalized pi electrons along the double conjugated bonds system. These features allow to determine in a semiquantitative way the optical absorption spectrum of the molecules by means of a simple model of free particles (the pi electrons) constrained to move inside a one-dimensional infinite potential well with length equal to the length of the double conjugated bonds system.
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| Beta-carotene molecule |
Lets consider the case of the beta-carotene, its real optical absorption spectrum is like the one shown in the figure. Beta-carotene absorbs the light with wavelengths around 470 nm, it means that beta-carotene preferably absorbs blue light, but the yellow, orange and red light is almost not absorbed by the molecules (the light that is not absorbed by the beta-carotene gives place to the orange color of the pumpkins).
Consideremos el caso de la molécula de beta-caroteno, su espectro de absorción óptica real es como el que se muestra en la figura. El beta-caroteno absorbe la luz de longitudes de onda cercanas a 470 nm, esto significa que el beta-caroteno absorbe preferentemente luz azul, pero la luz amarilla, naranja y roja prácticamente no es absorbida por las moléculas (la luz que no es absorbida por el beta-caroteno da lugar al color naranja de las calabazas).
Considering that the beta-carotene double conjugated bonds system is formed by 22 carbon atoms (and 22 pi electrons), and that the length of the system is equal to 18.5 angstroms, then the wavelength of light corresponding to the energy necessary to excite one electron from the Highest Ocuppied Molecular Orbital (energy level n=11 - according to the Pauli exclusion principle there are only two pi electrons for each energy level) to the Lowest Unocuppied Molecular Orbital (energy level n=12) is equal to 490 nm. The so calculated value is a good aproximation to the real one, however, the real spectrum does not consist exclusively of only one wavelength because the molecular vibrational and rotational contributions.
Considerando que el sistema de enlaces dobles conjugados del
beta-caroteno está formado por 22 átomos de carbono (y 22 electrones pi), y que la longitud del sistema es igual a 18.5 angstroms,
entonces la longitud de onda de la luz correspondiente a la energía
necesaria para excitar un electrón del Más Energético de los Orbitales Moleculares Ocupados (nivel de energía n=11 - de acuerdo con el principio de exclusión de Pauli sólo hay dos electrones pi por cada nivel de energía) al Orbital Molecular Desocupado Menos Energético (nivel de
energía n=12) es igual a 490 nm. El valor así calculado es una buena
aproximación al valor real, sin embargo, el espectro verdadero no consiste
exclusivamente de una sóla longitud de onda, debido a las contribuciones vibracionales y rotacionales de las moléculas. 
