I want to find a manga (maybe black and white) that I read last year.
It tells the story of a hero (MC) who was fired/kicked from his party after they saw that he fights more like a cruel devil than a hero. He meets a former demon leader (Heroine) who was kicked out from the Devil Lord's castle and bullied by his troops.
The main character is a man who has white hair. I don't remember too much about the heroine, but she might have had long black hair and glasses.
This may be A Former Child Soldier Who Uses a Magic Sword Wants to Live with An Older Sister of a Former Enemy Executive. Many details match up, but the timeline doesn't. According to mangaupdates, it was released in 2020, but no (English) translations were released before May 2021.
Seto is an ex-boy soldier who was expelled from the party of heroes on his journey because of his ruthless and incomparable fighting style. The former leader of the Demon King’s army, Satis, was kicked out after a series of failures.
The two “former” enemies decide to travel together to find a second life.
The main character has white hair in the b&w panels (it looks silverish on the cover). The heroine has glasses, but red hair - but again, it's a black&white manga, so it appears dark-grey.
Might this be The Rise of the Unemployed Wise Man?
Karna was a boy that was given the title of "Wiseman" from heaven, is scouted by the hero party that had great expectations for his skills.
However, his skill "necromancer" was creepy and he was immediately exiled.
Karna, who got lost on the road without getting a reward, had accidentally invaded the Demon King territory and was targeted by the demons! He faced a demon with mighty power but defeats it with magic that overwhelmed it by far.
Meanwhile, Cecilia the demon, who saw the whole scene, offers Karna a large annual salary. And thus Karna decided to become an enemy of humanity as a magician with the Demon King army.
The main character is a young necromancer with blond hair, which looks white on the black & white pages. He gets kicked out of the hero's party though, because some of the other members of the party regard his skills as being "disgusting" (in reality though, it appears that at least part of the reason they don't like him is because he's so powerful that he makes them feel useless).
He subsequently encounters a female demon lord with long black hair, who is impressed by his powers, and hires him for a generous salary. There's also a young male demon he meets who is trying to win the favour of the female demon lord. He might be the character you mentioned who was bullied by his troops (a pair of demons do kind of make fun of him at one point).
