What do the different shades portray in active trader volume bars on thinkorswim desk

[techphets]

I'm using thinkorswim desktop and there is something I have been unable to find documentation for.

Notice the darker portion of the volume bar, circled in red in the image below. Near the end of a trading day there are usually several different shades in some of the bars while other bars are only dark or only light.

Does anyone know what this represents?

Also, if you happen to know where I could find this documented in either a text documentation or a video, please let me know.

Thanks,

David

[MC]

In tradestation's DOM it was showing the age of activity. So the darker volume is probably older and the lighter is probably more recent.

Just my guess anyhow...