Show tow times change map with same color legend with ggalluvial

I have a map with 3 classes. I evaluated size of classes in two times. I am using dplyr, ggplot and ggalluvial libraries to present changes from 'First' time to 'Second' time.

data.frame(
First  = c("A", "B", "C","A","A"),
Second = c("A", "B", "C","B","C"),
Size   = c(83,  61,  32,  28, 16)
) %>%
ggplot(
  aes(
    axis1 = First,
    axis2 = Second,
    y = Size ,
    fill = Second )) +
geom_alluvium(aes(fill = Second )) +
geom_stratum() +
geom_text(
  stat = "stratum", 
  size = 5,
  aes(label = after_stat(stratum)))  +
theme_void() +
theme(legend.position = "none") 

But I have this figure

Ggalluvial shows no change classes with same color and clases with changes with grey color.

I need to present both axis with same color legend.

How could I do that?

You are trying to map the fill color of the first axis according to the value of Second, which doesn't make sense (there are three values for Second where First == A). You should map the stratum fill to after_stat(stratum), as you do for the geom_text labels:

data.frame(
  First  = c("A", "B", "C","A","A"),
  Second = c("A", "B", "C","B","C"),
  Size   = c(83,  61,  32,  28, 16)
) %>%
  ggplot(
    aes(
      axis1 = First,
      axis2 = Second,
      y = Size)) +
  geom_alluvium(aes(fill = Second)) +
  geom_stratum(aes(fill = after_stat(stratum))) +
  geom_text(
    stat = "stratum", 
    size = 5,
    aes(label = after_stat(stratum)))  +
  theme_void() +
  theme(legend.position = "none")