Basically trying to replicate something like this:
I would the to use something like this to create a plot that has different colours above and below 0:
Plot.areaY(data, {
x: "month",
y: "value",
fill: d => d["value"] > 0 ? "green" : "red", // Use a function for fill
fillOpacity: 0.1 // Sets the fill opacity of the area under the line plot
}),