library(shiny)
shinyApp(ui = fluidPage(
  sidebarPanel(
    selectInput(inputId = "dropdown", label = "Select data set:",
                choices = c("iris", "mtcars"), selected = "iris")
  ),

  mainPanel(fluidPage(
    fluidRow(plotOutput("plot"),
             actionButton("color", "Color"))
  ))
), server = function(input, output) {
  get_data <- reactive({
    if(input$dropdown == "iris") {
      return(list(dat = iris, x = "Sepal.Length", color = "Species"))
    } else {
      return(list(dat = mtcars, x = "mpg", color = "cyl"))
    }
  })
  output$plot <- renderPlot({
    dat <- get_data()
    return(plot(dat$dat[, dat$x]))
  })

  observeEvent(input$color, {
    output$plot <- renderPlot({
      dat <- get_data()
      return(plot(dat$dat[, dat$x], col = dat$dat[, dat$color]))
    })
  })
})