icon property
icon Icon of the topic. Icon color must be one of 0x6FB9F0, 0xFFD67E, 0xCB86DB, 0x8EEE98, 0xFF93B2, or 0xFB6F5F. Telegram Premium users can use any custom emoji as topic icon, other users can use only a custom emoji returned by getForumTopicDefaultIcons
Implementation
final ForumTopicIcon icon;