The Collaborative International Dictionary of English v.0.48:

Redden \Red"den\ (r?d"d'n), v. t. [imp. & p. p. Reddened
   (-d'nd); p. pr. & vb. n. Reddening.] [From Red, a.]
   To make red or somewhat red; to give a red color to.
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The Collaborative International Dictionary of English v.0.48:

Redden \Red"den\, v. i.
   To grow or become red; to blush.
   [1913 Webster]

         Appius reddens at each word you speak.   --Pope.
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         He no sooner saw that her eye glistened and her cheek
         reddened than his obstinacy was at once subbued. --Sir
                                                  W. SCott.
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WordNet (r) 3.0 (2006):

redden
    v 1: turn red, as if in embarrassment or shame; "The girl
         blushed when a young man whistled as she walked by" [syn:
         blush, crimson, flush, redden]
    2: make red; "The setting sun reddened the sky"
    3: turn red or redder; "The sky reddened"