Although in English we know that adjectives only have a (singular) form regardless of whether the noun to which it belongs is singular or plural, in <u>Spanish the case is completely contrary, adjectives have the number and often gender</u>.
Example: <em>Rojo</em>.
Roja: female and singular.
Rojo: male and singular.
Rojas: female and plural.
<u>Rojos</u>: male and plural.
How <em>"bolígrafos"</em> is a male and plural noun, the right form of the adjective is <u><em>"rojos"</em></u>.
<em>Tengo unos bolígrafos </em><u><em>rojos</em></u><em>.</em>