It's not colored and it looks more like a manga than a manhwa, but this might be the closest to Steel Under Silk in terms of premise, minus the political subplot.
https://www.mangago.me/read-manga/to_take_an_enemys_heart/
This one is not historical, but what makes me think of them as similar is the relationship between the MCs. There's a power dynamic, they don't really like each other + some things that are spoilers. There's also a plot included, mystery and scheming.
https://www.mangago.me/read-manga/the_foul/
thank you so much!!
Np. Hope you'll enjoy at least one of them. ^^
Toxin!!! - https://www.mangago.me/read-manga/toxin/
any manhwas similar to steel under silk please