Rule 1: 1. Form IX اِفْعَلَّ: colours and defects
Every colour or defect adjective of the pattern أَفْعَلُ / فَعْلَاءُ has a Form IX verb meaning to be, become or turn that colour: أَحْمَرُ / اِحْمَرَّ, أَخْضَرُ / اِخْضَرَّ, أَصْفَرُ / اِصْفَرَّ, أَزْرَقُ / اِزْرَقَّ, أَبْيَضُ / اِبْيَضَّ, أَسْوَدُ / اِسْوَدَّ, أَسْمَرُ / اِسْمَرَّ to turn brown; and for defects أَعْوَجُ / اِعْوَجَّ to be crooked, أَحْوَلُ / اِحْوَلَّ to squint, أَطْرَشُ / اِطْرَشَّ to go deaf. Form IX is used for nothing else; it is always intransitive, and therefore it has no passive. Verbal noun اِفْعِلَالٌ: اِحْمِرَارٌ redness, blushing; participle مُفْعَلٌّ: مُحْمَرٌّ reddened.
اِحْمَرَّتِ السَّمَاءُ عِنْدَ الْغُرُوبِ.
iḥmarrati s-samāʾu ʿinda l-ghurūbi
The sky turned red at sunset.
يَخْضَرُّ الْوَادِي بَعْدَ الْمَطَرِ.
yakhḍarru l-wādī baʿda l-maṭari
The valley turns green after the rain.
اِصْفَرَّتْ أَوْرَاقُ الشَّجَرِ فِي الْخَرِيفِ ثُمَّ تَسَاقَطَتْ.
iṣfarrat awrāqu sh-shajari fi l-kharīfi thumma tasāqaṭat
The leaves of the trees turned yellow in autumn, then fell one after another.
اِبْيَضَّتْ قِمَمُ الْجِبَالِ بِالثَّلْجِ فِي الشِّتَاءِ.
ibyaḍḍat qimamu l-jibāli bi-th-thalji fi sh-shitāʾi
The mountain tops turned white with snow in winter.
اِسْوَدَّ السَّحَابُ فَعَرَفْنَا أَنَّ الْمَطَرَ قَرِيبٌ.
iswadda s-saḥābu fa-ʿarafnā anna l-maṭara qarībun
The clouds turned black, and we knew that rain was near.
اِعْوَجَّ غُصْنُ الشَّجَرَةِ مِنَ الرِّيحِ.
iʿwajja ghuṣnu sh-shajarati mina r-rīḥi
The branch of the tree was bent by the wind.
A defect, not a colour: the same form.