[Xy-pic] defining color (xypdf, xcolor)

[Thierry Masson]

Hello,
I would like to create a macro to define a new xycolor from a color 
defined by the very powerful package xcolor. I have tried the 
following:

\def\extractcolorspecxy#1#2{\XC at split{#1}\aftergroupdef#2{\@@clr}}%
\def\@putawaycomma#1,#2,#3{#1 #2 #3}%
\def\createxycolorforcolor#1{%
\extractcolorspecxy{#1}{\@spec}%
\newxyColor{#1}{\expandafter\@putawaycomma\@spec}{rgb}{}\relax
}%

First line is a modification of a macro in the xcolor package. It 
extracts the color specifications (I ALWAYS suppose the color to be in 
the rgb model here), and put it in a macro. This macro expands to 3 
numbers (r,g,b) separed by commas. I defined the macro in the second 
line to put away the commas... The last definition, lines 3-5, creates 
a color for xypic with a macro from the xycolor package.

What surprised me is that it works! When I write in my latex file
\colorlet{myred}[rgb]{red!50!white} % new color from the xcolor package 
in the rgb model
\createxycolorforcolor{myred} % same color for use in xypic

then I can use "myred" to colorize arrows in my xymatrix ( 
\ar@[myred][d] ).
BUT, if I put a second definition in my latex file, with an other name 
for my new color, the first color is replaced by the second one!
This I don't understand ! I can't define two colors with my definition. 
Why???

I use pdflatex, and the package xypdf.

(Purpose of this: I use the package Beamer to create presentations with 
pdflatex and I want the arrows in some diagrams to have the same color 
as some nodes. I chose my color to fade away what is not the purpose of 
my explanation, or on the contrary to reveal some parts of the diagram. 
Very efficient!)

Thanks for any advice.
Thierry