You can change the color of the graph by adding a colour argument inside visreg2d function. You can for example use one of the standard R palettes such as terrain.colors()
or heat.colors()
or modify/create your own palettes e.g. with the package RColorBrewer (as in the example attached).
visreg2d(model, x="RELATEDNESS", y = "Hs_obs", scale = "response",
xlab = "Relatedness", ylab = "Heterozygosity", main = "Flower abundance",
zlim = c(0,100), col = colorRampPalette(brewer.pal(9,"Reds"))(20))