colorplaner | Visualize Two Variables Per Color Aesthetic through Color | Data Visualization library

I'm trying to plot a gradient fill map of all counties in the United States using the following code. I'd like to have a continuous darkblue -> darkred gradient fill for the map fill, but for some reason my code isn't doing this even though I added:

I'm trying to plot two variables taken from raster datasets on a map in R to produce something that looks a bit like this:

However, ideally I'd like the scale from bottom left to top right to be in greyscale (from light grey to black) thus highlighting areas where there is little divergence in the two variables.

So far this is what I have so far using the package colorplaner:

I have gotten a choropleth scripted for US county level data, but upon plotting it am finding it hard to read. The counties can get very crowded and begin blending in to each other - it becomes difficult to tell where one state ends and another begins. Does anyone know a method of plotting a choropleth map by county data and creating border outlines by state? Here is the code I have so far for reproduction: