LESSCSS: interpolation of a mixin argument - less

Question

I have the following code:
#red_SOPRA: orange;
#red: red;
.gradient(#color) when (#color = red)
{
background: linear-gradient( 180deg, #color 0%,yellow 100%);
}
div
{
.gradient(red);
}
Compiled into:
div {
background: linear-gradient(180deg, #ff0000 0%, #ffff00 100%);
}
I would like to add "_SOPRA" to the end of variable "#color" present in background definition, in order to obtain an interpolated and dynamic name of variable used in mixin.
I tried with ## and #{color} definitions but without success.
How to obtain a generated background like this (with value "#ffa500" - #red_SOPRA value - instead of "#ff0000")?
div {
background: linear-gradient(180deg, #ffa500 0%, #ffff00 100%);
}

Answer 1

Option #1
Unless I'm missing something from what you are trying to achieve, then simply removing the guard expression when (#color = red) will get a "dynamic" value output for #color, allowing it to be called directly by #red_SOPRA or #blu_SOPRA (whatever you pass).
LESS
#red_SOPRA: orange;
#red: red;
.gradient(#color)
{
background: linear-gradient( 180deg, #color 0%, yellow 100%);
}
div {
.gradient(#red_SOPRA);
}
CSS Output
As you wanted...
div {
background: linear-gradient(180deg, #ffa500 0%, #ffff00 100%);
}
Option 2
You can set up the mixin to perform a "'merging' between string passed" and the "_SOPRA" suffix, something like this:
LESS
#red_SOPRA: orange;
#blu_SOPRA: blue;
.gradient(#pre; #SOPRA: ~'#{pre}_SOPRA'; #color: ##SOPRA)
{
background: linear-gradient( 180deg, #color 0%,yellow 100%);
}
div
{
.gradient('red');
}
.test {
.gradient('blu');
}
CSS Output
div {
background: linear-gradient(180deg, #ffa500 0%, #ffff00 100%);
}
.test {
background: linear-gradient(180deg, #0000ff 0%, #ffff00 100%);
}
This will not fail gracefully if #color does not resolve to a valid ##SOPRA value.

Answer 2

First of all, I would like to thank ScottS for his answer, especially second option that is what I needed.
Secondly, I add solution that finally yesterday I found by myself, very similar in philosophy but little different and (maybe) easier to understand by newby, due to a temp variable.
#red_SOPRA: orange;
#red: red;
.gradient(#color) when (#color = 'red')
{
#stop-1:~"#{color}_SOPRA";
background: linear-gradient( 180deg, ##stop-1 0%,yellow 100%);
}
div
{
.gradient('red');
}
Philosophy is the same, but with this "double passage" I think that could be more "talking"... :-)
UPDATE:
After correct comment of seven-phases-max, I updaded my solution. Please note that parameter must be passed as string with 'red'