The Collaborative International Dictionary of English v.0.48:

Sorrel \Sor"rel\, a. [F. saur, saure, OF. sor, sore, probably of
   Teutonic origin; cf. D. zoor dry, LG. soor; the meaning
   probably coming from the color of dry leaves. See Sear, a.,
   and cf. Sorel.]
   Of a yellowish or redish brown color; as, a sorrel horse.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sorrel \Sor"rel\, n.
   A yellowish or redish brown color.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Sorrel \Sor"rel\, n. [F. surelle, fr. sur sour, fr. OHG. s?r
   sour. See Sour.] (Bot.)
   One of various plants having a sour juice; especially, a
   plant of the genus Rumex, as Rumex Acetosa, Rumex
   Acetosella, etc.
   [1913 Webster]

   Mountain sorrel. (Bot.) See under Mountain.

   Red sorrel. (Bot.)
   (a) A malvaceous plant (Hibiscus Sabdariffa) whose acid
       calyxes and capsules are used in the West Indies for
       making tarts and acid drinks.
   (b) A troublesome weed (Rumex Acetosella), also called
       sheep sorrel.

   Salt of sorrel (Chem.), binoxalate of potassa; -- so called
      because obtained from the juice of Rumex Acetosella, or
      Rumex Axetosa.

   Sorrel tree (Bot.), a small ericaceous tree (Oxydendrum
      arboreum) whose leaves resemble those of the peach and
      have a sour taste. It is common along the Alleghanies.
      Called also sourwood.

   Wood sorrel (Bot.), any plant of the genus Oxalis.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sorrel
    adj 1: of a light brownish color [syn: sorrel, brownish-
           orange]
    n 1: any plant or flower of the genus Oxalis [syn: oxalis,
         sorrel, wood sorrel]
    2: any of certain coarse weedy plants with long taproots,
       sometimes used as table greens or in folk medicine [syn:
       dock, sorrel, sour grass]
    3: East Indian sparsely prickly annual herb or perennial
       subshrub widely cultivated for its fleshy calyxes used in
       tarts and jelly and for its bast fiber [syn: roselle,
       rozelle, sorrel, red sorrel, Jamaica sorrel,
       Hibiscus sabdariffa]
    4: large sour-tasting arrowhead-shaped leaves used in salads and
       sauces [syn: sorrel, common sorrel]
    5: a horse of a brownish orange to light brown color

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "sorrel":
   bay, bayard, beige, brown, brownish, brownish-yellow, brunet,
   buckskin, calico pony, chestnut, chocolate, cinnamon, cocoa,
   cocoa-brown, coffee, coffee-brown, dapple-gray, drab, dun,
   dun-brown, dun-drab, ecru, fawn, fawn-colored, fuscous, gray,
   grege, grizzle, hazel, khaki, lurid, nut-brown, olive-brown,
   olive-drab, paint, painted pony, piebald, pinto, roan, seal,
   seal-brown, sepia, skewbald, snuff-colored, tan, taupe, tawny,
   toast, toast-brown, umber, umber-colored, walnut, walnut-brown,
   yellowish-brown