Comment 1 for bug 1078287

> But if I create a new object, take the color picker and assign it the
> defined color, Inkscape still uses the RGBA notation for the new object
> fill instead of referencing the color url.

To be expected (and not a bug): '<solidColor>' is not part of current the SVG 1.1. specification [1] (Solid color paint servers have been accepted for SVG 2.0, and are defined in the initial draft version of the spec for SVG 2.0 [2]. This special paint server is also part of the SVG Tiny 1.2 specification [3] which Inkscape does not support [4]).

With current stable Inkscape 0.48, you can use custom swatches for named colors in the current document:
<http://wiki.inkscape.org/wiki/index.php/Release_notes/0.48#Custom_Swatches>
<http://tavmjong.free.fr/INKSCAPE/MANUAL/html/Attributes-Fill-Stroke.html#Attributes-CustomSwatches>

[1] <http://www.w3.org/TR/SVG11/color.html>
[2] <http://www.w3.org/TR/2012/WD-SVG2-20120828/pservers.html#SolidColors>
[3] <http://www.w3.org/TR/SVGMobile12/painting.html#SolidColorElement>
[4] Bug #171283 “Implement SVG Tiny profile”
    Bug #701406 “Request feature - suport svgb format”