To see what colours in the shadows are clipping to zero, and exactly where they occur in your image, you can hold down the Alt (Windows) or Option (Mac) key and move the Blacks slider to the left (in ACR 7.x and 8.x).
To quote from the top of page 206 of Jeff Schewe's and Bruce Fraser's 'Real World Camera Raw with Adobe Photoshop CS5' (which deals with ACR 6.x - but AFAIAA nothing has changed in ACR 7.x or 8.x), you interpret the displayed result as follows:
"Unclipped pixels display as white. The other colors show you which channels are being clipped to level 0. Cyan pixels indicate red-channel clipping; magenta pixels indicate green-channel clipping; yellow pixels indicate blue-channel clipping. Red pixels indicate clipping in both green and blue channels; green pixels indicate clipping in red and blue channels; and blue pixels indicate clipping in red and green channels. Black pixels indicate that all three channels are clipped."
Does the same result hold true for the colour of the Histogram Shadow Clipping Indicator?
Yes, the same color scheme is used for the little "triangle clipping" indicators in the histogram. You can test this by using the per-channel point curve -- e.g., drag the bottom-left corner of the Red point curve to the right, so that you begin clipping the red channel in the shadows. You should see the left triangle indicator in the histogram (the shadow warning indicator) become colored red.
Thanks, Eric. I'll check out your suggestion about the point curve.
I've already started to do some testing of my own, based on the Histogram's reaction faced with simple images composed only of various combinations of pure (fully saturated) Black, White, Grey (dark, mid and light), Red, Green, Blue, Cyan, Magenta and Yellow. The results so far are a little surprising, but I'll report in full when I've had the chance to complete the test.