setSliderColor(color, sliderId)

if (interactive()) {

library(shiny)
library(shinyWidgets)

ui <- fluidPage(

  # only customize the 2 first sliders and the last one
  # the color of the third one is empty
  setSliderColor(c("DeepPink ", "#FF4500", "", "Teal"), c(1, 2, 4)),
  sliderInput("obs", "My pink slider:",
              min = 0, max = 100, value = 50
  ),
  sliderInput("obs2", "My orange slider:",
              min = 0, max = 100, value = 50
  ),
  sliderInput("obs3", "My basic slider:",
              min = 0, max = 100, value = 50
  ),
  sliderInput("obs3", "My teal slider:",
              min = 0, max = 100, value = 50
  ),
  plotOutput("distPlot")
)

server <- function(input, output) {

  output$distPlot <- renderPlot({
    hist(rnorm(input$obs))
  })
}

shinyApp(ui, server)

}