Signed colour values and clamping

I'd like to use signed vertex colour values in the range -0.5 to 0.5
with an additive texture stage to create high-lights and low-lights.
Unfortunately it seems that the colour values are clamped in the range 0
to 1 before input to the register-combiner.
 
The documentation says Neither floating-point nor signed integer values
are clamped to the range [0,1] before updating the current color.
However, color components are clamped to this range before they are
interpolated or written into a color buffer.
 
Is what I'm seeing consistent with the docs and expected behavour?
 
Thanks
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Robin E. Charlton                   [email protected]