viewUserName : String -> Html msg
viewUserName userName =
    span 
        [ Attr.css [ Css.color (Css.hex "#222") ] ]
        [ text userName ]

viewAge : Int -> Html msg
viewAge age =
    span 
        [ Attr.css [ Css.color (Css.hex "#333") ] ]
        [ text (String.from Int age) ]

import Ui.Color as Color

viewUserName : String -> Html msg
viewUserName userName =
    span 
        [ Attr.css [ Css.color Color.darkGrey ] ]
        [ text userName ]

viewAge : Int -> Html msg
viewAge age =
    span 
        [ Attr.css [ Css.color Color.darkGrey ] ]
        [ text (String.from Int age) ]

module Ui.Color exposing (darkGrey, andOtherColors)

import Css

darkGrey : Color
darkGrey =
  Css.hex "#222"

view : Model -> Html Msg
view model =
    div []
        [ ...
        , Button.button UserAddedItem "Add"
            |> Button.withColor (Css.hex "00FF00")
            |> Button.toHtml
        ]

-- LintConfig.elm
module LintConfig exposing (config)

import Lint.Rule exposing (Rule)
import NoDefiningColorsOutsideOfUiColor


config : List Rule
config =
    [ NoDefiningColorsOutsideOfUiColor.rule
    ]

cd <your-project>
# Creates lint/elm.json and lint/LintConfig.elm
elm-lint init

rule : Rule
rule =
    Rule.newSchema "NoDefiningColorsOutsideOfUiColor"
        |> Rule.withSimpleExpressionVisitor expressionVisitor
        |> Rule.fromSchema

expressionVisitor : Node Expression -> List Error
expressionVisitor node =
    case Node.value node of
        -- "Application" means function call
        Expression.Application (function :: arguments) ->
            case Node.value function of
                -- "FunctionOrValue" means a variable
                Expression.FunctionOrValue [ "Css" ] "hex" ->
                    [ Rule.error
                        { message = "Do not define colors outside of Ui.Color"
                        , details =
                            [ "At fruits.com, we try to have all the colors in our application defined in the Ui.Color file. This helps us to have a consistent color palette across the application."
                            , "You should define this color in the Ui.Color module, and import it to use it at this location. Do check whether the color does not already exist though."
                            ]
                        }
                        (Node.range node)
                    ]
                _ ->
                    []
        _ ->
            []