The English suffix -ish is a diminutive. When added to a root, it gives an approximate meaning. For example, the word "eightish" means around eight and "yellowish" means close to yellow. In Spanish, approximate colours aren't formed with an invariable suffix as in English.
Here are ten approximate colours in Spanish:
red reddish rojo rojizo
blue bluish azul azulejo
yellow yellowish amarillo amarillento
black blackish negro negruzco
white whitish blanco blancuzco
pink pinkish rosa rosáceo
green greenish verde verdoso
orange orangish naranja anaranjado
brown brownish pardo pardusco
grey greyish gris grisáceo
Compared to English, we notice that approximate colours in Spanish aren't formed with a single suffix form. With the colours black and white, the word final -o becomes a -u and the suffix -zco is added. In the case of brown, the suffix is spelt -sco. The colours grey and pink add the suffix variant -áceo. The other colours are formed with different variants.
To form approximate colours in English, the rule is simple. The suffix -ish is added to the root. In Spanish, however, approximate colours are formed with a single suffix, but this suffix has many variants.