As far as I know there is no color contrast defined in the standard nor a
border standard, the entire DW needs to meet the shade contrast - light on dark
or dark on light. As long as the DW is appropriately contrasted to the
sidewalk they can both be red, its just that one needs to be light red and the
other needs to be dark red.

Research completed by FHWA for the Access Board in 2006 (see it online at
www.access-board.gov/research/dw-fhwa/report.htm) suggests that a contrasting
border must be at least 6 inches wide for detectability.

Even though the research posted on the Access Board site found that a border
was visually detected from a distance, that research did not consider the
issues of a person walking over the border on a slope, or what the border was
seen as. People were only asked to tell the researcher when they saw it, not
to identify what they saw, or to walk across the surface.
I continue to have concerns that such a border (6 inches wide) will be seen as
a curb, step, or other level change by low vision or elderly persons, who have
more difficulty with depth perception generally.