I am not certain if this is intended, but the "rgbi:" specifications
appear to tolerate whitespace

       (color-values-from-color-spec "rgbi:0/0/ 0")

despite not being documented.  From what I see, the issue stems from
parse_float_color_comp calling strtod, where strtod(3) says:

       The  expected  form of the (initial portion of the) string is op‐
       tional leading white space as recognized by  isspace(3) [...]

If this is considered to be an issue (I'd argue that relying a detail of
this kind in the libc could lead to problem), something like this could
solve the issue: