This is the 736th most frequent Spanish word.
amarilla
"Amarilla" means "yellow" in English.
In this sentence, 'amarilla' is used as an adjective to describe the color of the noun 'camisa', agreeing in gender and number.
La camisa es amarilla.
The shirt is yellow.
Here, 'amarilla' is modifying the noun 'señal', again agreeing in gender and demonstrating adjectival agreement.
Hay una señal amarilla en la calle.
There is a yellow sign on the street.
In this context, the adjective 'amarillo' (masculine form of 'amarilla') describes the resulting color of the sky, agreeing in gender with 'cielo'.