Why is the colour green connected with islam? Why Al-Khidr was callled "The green One"? Why favourite colour of prophet Mohamed was the colour green?
Green serves as a recurring color-theme throughout the Quran. It's the color of many items in Heaven (cushions, clothes, gardens, etc) It's the color used in reference to the restored earth after being dead and so on. As for the Prophet (PBUH), he wore many green garments, saw the vision of Aisha (RA) wrapped in green, and so on.
It's also important to note, that for Arabs of the time who were living in dry dead deserted areas around Arabia, the Green Color serves as a symbol of life, hope, desire, etc.
Some verses and Ahadith:
Reclining on green cushions and beautiful carpets. (55:76)
Sent down rain from the sky and the earth becomes green? Indeed, Allah is Subtle and Acquainted. (22:63)
Two Gardens, dark green and fresh (55:64)
It was narrated that Abu Rimthah said: "I saw the Prophet (ﷺ) delivering the Khutbah, wearing two green Burds." ( Jami` at-Tirmidhi, Chapters on Manners )
Narrated 'Aishah: that Jibril came to the Prophet (ﷺ) with her image upon a piece of green silk cloth, and he said: "This is your wife in the world, and in the Hereafter." (Jami` at-Tirmidhi, Chapters on Virtues)
And Allah knows best.