Control blinking rhythms by changing pause durations.
Use a loop to alternate between turning the RGB LED A on or off. Control the rhythm of the blinking by changing the duration of the pauses.
basic.forever(function () {
    sb.setRgbLedColor(sb.rgbLed(SBRgbLed.RgbLedA), sb.color(SBColor.White))
    basic.pause(1000)
    sb.setRgbLedColor(sb.rgbLed(SBRgbLed.RgbLedA), sb.color(SBColor.Black))
    basic.pause(1000)
})If using micro:bit for the first time, follow this setup.