I have a spatial polygon data-frame and I am trying to plot a heatmap via spplot. It's a map of Europe and the values for the heatmap are in a column of the data-frame. Plotting it works fine but I have some NAs in the column used for the colors and these countries are just not colored, so they are white. I'd really like them to be grey, however.. I have no idea how I can do this in spplot.. can anyone help?