Barcoloring a series with barcolor

The annotation function barcolor lets you specify a color for a bar dependent on the fulfillment of a certain condition. The following example script renders the inside and outside bars in different colors:

study("barcolor example", overlay=true)
isUp() => close > open
isDown() => close <= open
isOutsideUp() => high > high[1] and low < low[1] and isUp()
isOutsideDown() => high > high[1] and low < low[1] and isDown()
isInside() => high < high[1] and low > low[1]
barcolor(isInside() ? yellow : isOutsideUp() ? aqua : isOutsideDown() ? purple : na)

As you can see, when passing the na value, the colors stay the default chart color.