Concatenate Map values when providing several keys

I’m on my learning curve of Elixir with Exercism.
I have a question I’m stuck with:

• given an list of colors like colors = [:orange, :orange, :black]
• and a color/value Map defined as follows:

@colors_map %{
    black: 0,
    brown: 1,
    red: 2,
    orange: 3,
    yellow: 4,
    green: 5,
    blue: 6,
    violet: 7,
    grey: 8,
    white: 9
  }

My first try was to iterate on the colors list and call Map.get/3 function:

Enum.each(colors, fn c -> Map.get(@colors_map, c) end)

But I’m stuck with how to concatenate the values extracted from the Map as a number resulting from the concatenation of the values for the first 2 list values only.
For example, for the colors = [:orange, :orange, :black], I’d like to get:
33 (what corresponds to twice 3 for the :orange key, `:black being the last value is dropped).

In short, I need to convert the provided list of colors to its numeric resistance value.

Update:
I alsotried this:

digits = Enum.take(colors, 2)
for x <- digits, do: Map.get(@colors_map, x)

This returns me a list [3, 3]. How is it possible to call join on the result list in case of the use of the comprehension?

Thank you.