Well "pêche" (fishing) and "pêche" (peach) have both a plural form :
Ces pêches sont très juteuses.
Il existe plusieurs types de pêches en bateau.
But the color "pêche" has no plural form because it is an adjective derived from a name.
Like Greg said :
Color adjectives that are derived from a noun (pêche, orange, marron, etc) are invariable (with some exceptions such as "rose"). Ex: des chemises pêche.
