Repetition.

set2d.elm

import Array exposing (Array)

type alias Model = Array (Array Color)
type alias Point = (Int, Int)
type Color = Red | Green | Blue | Purple

set2d : Point -> Color -> Model -> Model
set2d (x, y) color model =
  Array.get y model
    |> Maybe.map (flip (Array.set y << Array.set x color) model)
    |> Maybe.withDefault model