Change the color and font of text in Shiny App

You can use css as @jbaums indicated

library(shiny)
runApp(list(
  ui = bootstrapPage(
    numericInput('n', 'Number of obs', 100),
    textOutput('text1'),
    tags$head(tags$style("#text1{color: red;
                                 font-size: 20px;
                                 font-style: italic;
                                 }"
                         )
              )
  ),
  server = function(input, output) {
    output$text1 <- renderText({ paste("hello input is",input$n) })
  }
))

Normally you would include this in a styles.css file but it is shown inline here to be self contained. #text1 refers to the DOM element with id=text1 and the contents of the curly brackets are the relevant styles.

If only want to change a certain part of the returning string, one can use htmlOutput instead of textOutput

On server side just return

output$text1 <- renderText({ paste("hello input is","<font color=\"#FF0000\"><b>", input$n, "</b></font>") })

In this way, Shiny UI will perform HTML.

in ui.r:

span(textOutput("message"), style="color:red")

in server.r:

output$message <- renderText({"This is some red text"})