Is wearing certain colors of underwear, home décor, or an isdal considered forbidden if a man who saw the woman in her underwear might recall the situation upon seeing her in the same colors in the street? And must she wear a niqab to avoid him seeing her and recalling the situation?

There is no harm in wearing modest clothing of any color that does not attract attention. What you mention regarding your fears are whispers (waswas) that should be ignored, and you should be wary of succumbing to them. What helps in getting rid of these whispers is seeking Allah's help, sincerely turning to Him, and supplicating frequently.