how to change colour in yaml file

[bbucior]

I don't know of a way to set it from the yaml header, but you can customize it from the custom.css file or an RMarkdown block:

---

custom.css:

:root {
    --header-color: #008040;
    --other-color-for-h2-gradient: #008080;
}

---

Or within your poster's .Rmd file (I'm less familiar with this method):

```{css, echo=FALSE}
:root {
    --header-color: #408040;
    --other-color-for-h2-gradient: #408080;
}