I tried with this
import panel as pn
pn.extension()
tmpl = pn.template.VanillaTemplate(title='color')
tmpl.header_background = 'red'
tmpl.header_color = 'blue'
tmpl.main.append(pn.Row())
tmpl.show()
header background is working, but header color not
it seems it gets overriden
one solution is to use the important command in raw_css
import panel as pn
css = """
.title{color:blue !important; }
"""
pn.extension(raw_css=[css])
tmpl = pn.template.VanillaTemplate(title='color', header_color = 'blue')
tmpl.header_background = 'red'
tmpl.main.append(pn.Row())
tmpl.show()