"amareloS"....is the "s" something to do with plural?
Yeah. Colors change in number and gender.
Be careful: some colors have no gender: marrom, cinza, rosa, verde, lilás, azul, laranja, carmesim, beige. etc.
Thank you. I have noticed that some of the colors with no gender also have no plural (ie; cinza), is this correct?
"colors" that are also used to refer to objects have no plural:
Pointing out: cinza = gray/ash, orange = laranja (color/fruit), rosa = pink/rose
Much information here: http://www.learningportuguese.co.uk/language/colours.html