This is the 3202nd most frequent Spanish word.
amarillo
Yellow.
Here, 'amarillo' is used as an adjective describing the color of 'el sol' (the sun).
El sol es de un color amarillo brillante.
The sun is of a bright yellow color.
'Amarillo,' in its 'amarilla' form, is used as an adjective to modify 'la flor' (the flower), agreeing in gender and number.
Me gusta la flor amarilla en el jardín.
I like the yellow flower in the garden.
'Amarillo' functions as a descriptor in a prepositional phrase indicating the color characteristic of 'los autobuses escolares' (school buses).