tmap Color in legend doesn't match the actual map

I tried to draw a map using tmap with the color palette in grey scale. However, I can not get the legend to match the actual color in the map.

Here are my codes:

 tmap::tm_shape(ut_master_T10) +
  tmap::tm_polygons(
    col = "t2d.18_64.censusrate.per1k.q5",
    palette = c("#FFFFFF", "#DEDEDE", "#BEBEBE", "#5F5F5F", "#000000"),
    alpha = 0.7,
    lwd = 0.5,
    title = "") +
  tmap::tm_layout(
    frame = FALSE,
    legend.outside = TRUE,
    legend.hist.width = 5,
    legend.text.size = 0.5,
    fontfamily = "Verdana") +
  tmap::tm_scale_bar(
    position = c("LEFT", "BOTTOM"),
    breaks = c(0, 10, 20),
    text.size = 0.5
    ) +
  tmap::tm_compass(position = c("LEFT", "TOP")) 

And here is the picture:

I tried using a different scale (ie viridis), and it is very clear that the color in the legend is lighter than in the actual map.

Can someone please help me?

Thank you!