RGB lighting is typically controlled by the motherboard software or a manufacturer specific RGB controller, if provided by your case manufacturer or fans/cooler manufacturer for example.
Post a screenshot of "selected as rainbow in the startup screen".
Ryzen R7 5700X | B550 Gaming X | 2x16GB G.Skill 3600 | Radeon RX 7900XT
Ryzen R7 5700G | B550 Gaming X | 2x8GB G.Skill 4000 | Radeon Vega 8 IGP
Ryzen R5 5600 | B550 Gaming Edge | 4x8GB G.Skill 3600 | Radeon RX 6800XT