template? mixin? template mixins? for modifying a struct setup

[Chris Katko]

given
```D
struct COLOR
{
float r, g, b, a; // a is alpha (opposite of transparency)
}

auto red   = COLOR(1,0,0,1);
auto green = COLOR(0,1,0,1);
auto blue  = COLOR(0,0,1,1);
auto white = COLOR(1,1,1,1);
//etc
```

is there a way to do:
```D
auto myColor = GREY!(0.5);
// where GREY!(0.5) becomes COLOR(0.5, 0.5, 0.5, 1.0);
```