根据随机值更改精灵帧

我有一个按钮列表(当前图像),我想根据列表中的值更改图像颜色

buttons = {"btn1","btn2","btn3","btn4"}

local buttonSheetData = {
    width = 150,
    height = 150,
    numFrames = 2,
    sheetContentWidth = 300,
    sheetcontentheight = 150,
}

local buttonSheet = graphics.newImageSheet("image/buttonSS.png", buttonSheetData)

local sequenceData = {
    {name = "black", start = 1, count = 1},
    {name = "red", start = 2, count = 1}
}

local btn1 = display.newSprite(buttonSheet, sequenceData)
btn1.x = 100
btn1.y = 90

local btn2 = display.newSprite(buttonSheet, sequenceData)
btn2.x = 200
btn2.y = 230

local btn3 = display.newSprite(buttonSheet, sequenceData)
btn3.x = 300
btn3.y = 90

local btn4 = display.newSprite(buttonSheet, sequenceData)
btn4.x = 400
btn4.y = 230

x = buttons[math.random(#buttons)] 
x:setFrame(2)

圆圈当前为黑色。每次运行代码时,我都希望它从列表中取一个随机值,并将颜色更改为红色。因此,当我运行代码时有一个不同的红色圆圈

我一直在犯错误: “试图调用方法'setSequence'(一个nill值)”