"redden" in the verb sense
1. blush, crimson, flush, redden
turn red, as if in embarrassment or shame
"The girl blushed when a young man whistled as she walked by"
2. redden
make red
"The setting sun reddened the sky"
3. redden
turn red or redder
"The sky reddened"
Source: WordNet® (An amazing lexical database of English)
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