R/quantmod: how to specify the bollinger bands colour?

Question

This could be more generally be How to change the theme colours? Or maybe TA colours are not controlled by theme?

This makes bollinger bands with a nice cloud effect:

chartSeries(bars, theme="white")
addBBands()

(See example of how it looks (near the bottom) )

The cloud effect is dark grey on this next example, so almost invisible.

chartSeries(bars, theme="black")
addBBands()

How do I change it to be, say, a nice bright red, with bright purple for the upper and lower lines? (Yeah, I know, -1 for the colour scheme)

I believe I'll be able to specify an 8-hex-digit colour to specify semi-transparency. But can I do anything more exotic? E.g. it would be rather cool to use a gradient and have it #ff0000 at the centre, fading to #330000 at the upper and lower lines. Is there any gradient support in quantmod charting?