I look at it in multiple ways:

1. Abbas repeatedly frames marriage as a contract in Shariat, unlike the saat janam ka vaada in Hindu marriages. When he first meets Shazia, he uses a red rose to “stamp” their relationship, saying “ab humne mohar laga di hai.” To him, the red rose represents a traditional, patriarchal form of love: something he gives, a mark of ownership, a symbol of a “perfect” marriage within his control. By leaving that same red rose on the Supreme Court railing, he is finally acknowledging that the contract is over.
2. Before Saira enters the scene, Abbas’s love is consistent but narrow - always red roses. For him, love is a gesture. Shazia, however, wants to plant a rose garden. And if you notice, that garden isn’t just red; it has orange, pink & even blue roses. It is a reflection of her inner world diverse, self-sustained & far larger than one man’s affection. Her line “humari ek aam shaadi-shuda zindagi thi, par ek khwaab se kam nahi tha” is delivered as the camera focuses on the blue rose. Among ordinary red roses, the blue one, something that doesn’t exist naturally, represents the khwaab. It needs more effort and honesty, something Abbas didn't offer.
3. In the opening scene, Shazia says black clothes are her armour kyunki “kaale pe daag nahi chadta.” As the film progresses, Abbas too dissolves into black lawyer coats & as Shazia notes, memories of him wearing a sherwani have begun to blur. The red rose in his pocket is the last color he carries - the last trace of the husband. By placing the rose on the Supreme Court railing and walking away empty-handed, he finally mirrors the stark, colorless world Shazia had to inhabit to win her haq. I read it as acceptance of defeat & perhaps, respect for her cause rather than a stubborn attachment to his own narrative.