The colour of the card label is specified using label_colour and recognises any valid colour string recognised by CSS. The input can either be a single colour, but you can also pass a vector as input assigning each card a different label colour:

galleries[1:3, ] %>% 
  cards(
    title = long_name,
    text = blurb,
    link = gallery_url,
    image = image_url,
    label_colour = c("#d6cadd", "white", "#c4d8e2")
  )
Ash Cloud and Blood

Simulated watercolours in ggplot2 using the transparent polygon method

Ghosts on Marble Paper

Abstract landscapes generated using ambient

Ice Floes

Abstract icebergs generated using ambient

For inset layouts, the colour needs to be partly transparent if you want the image underneath to show through:

galleries[1:3, ] %>% 
  cards(
    title = long_name,
    text = blurb,
    link = gallery_url,
    image = image_url,
    layout = "inset-bottom",
    label_colour = c("#d6caddaa", "#ffffffaa", "#c4d8e2aa"),
    breakpoint = 5
  )
Ash Cloud and Blood

Simulated watercolours in ggplot2 using the transparent polygon method

Ghosts on Marble Paper

Abstract landscapes generated using ambient

Ice Floes

Abstract icebergs generated using ambient