Plotly: Plotly express area chart with a single color for fill and line

Question

Hi i try to figure out how to use just one color per country - filled color. The following example gives a mix of transparent and a darker color per country. i expect just one color. Thanks for help

import plotly
#import plotly.graph_objects as go
import plotly.express as px
df = px.data.gapminder()
fig = px.area(df, x="year", y="pop", color="country",
          groupnorm='fraction')
plotly.io.write_image(fig, file='areatest.png', format='png')