ggridges color gradient per group

I didn't have the Catalan_elections data, so I'll try to provide a minimal example with dummy data.

For mapping multiple fill variables to different scales, you could use the ggnewscale package. A trade-off would be that you'd have to call seperate geom_density_ridges_gradient for each mapping.

library(ggplot2)
library(ggridges)
#> 
#> Attaching package: 'ggridges'
#> The following object is masked from 'package:ggplot2':
#> 
#>     scale_discrete_manual
library(ggnewscale)

df <- data.frame(
  x = c(rnorm(100, -2), rnorm(100, 2)),
  y = rep(LETTERS[1:2], each = 100)
)

ggplot(df, aes(x, y)) +
  geom_density_ridges_gradient(data = df[df$y == "A", ],
                               aes(fill = stat(x)), scale = 1) +
  scale_fill_gradient(low = "lightblue", high = "darkblue",
                      name = "A") +
  # Note that a fill scale must exist already before a new one can be used
  new_scale_fill() +
  geom_density_ridges_gradient(data = df[df$y == "B", ],
                               aes(fill = stat(x)), scale = 1) +
  scale_fill_gradient(low = "lightcoral", high = "darkred",
                      name = "B")
#> Picking joint bandwidth of 0.324
#> Picking joint bandwidth of 0.343

^{Created on 2019-12-03 by the reprex package (v0.3.0)}

I'm sure it would not be too difficult to extend this example to your own data.