R plot_ly() sf Choropleth Map, How to set NA or Zero values to a defined colour such as Grey

I am trying to find a solution to this solved python issue in R,

I am using an sf object to create a choropleth map using plotly as such:

plot_ly(postcodeelec, split = ~Postcode, color = ~Standard_E, alpha = 1, showlegend=F)

But some rows have NAs and therefore are outputted as transparent with different colours for the outline of that multipolygon.

Can anyone help?