Append string in LESS variable

As explained in comments, your problem is the recursive variable definition in the below line. Less does not support this as explained in this answer and this one.

@classes: "@{classes}, @{newClass}";

Based on your requirement explanation in comments (that there would be some extra padding etc when the colors are different), you could use one of the below methods.

Option 1: (will add the padding to every class and so repeated code)

.some-mixin(@newClass,@color,@color2){
      .@{newClass}{
         color: @color;
         & when not (@color = @color2){
            padding: 4px;
        }
      }
}

.some-mixin(abc,#ffffff,#000000);
.some-mixin(xyz,#ffffff,#ffffff);
.some-mixin(jkl,#ff00f0,#ff00ff);

The above Less would compile into below CSS:

.abc { 
  color: #ffffff;
  padding: 4px; /* colors were different */
}
.xyz {
  color: #ffffff;
}
.jkl {
  color: #ff00f0;
  padding: 4px; /* colors were different */
}

Option 2: (uses a dummy class + extend and so lesser code)

This option is probably what you are looking for as it avoids code repetition. We cannot extend a mixin and hence we use a dummy class. This should not be a big concern because it just adds one extra line to output CSS.

.common-padding-diff-color{ /* all styles that are needed when colors are different */
    padding: 4px;
}
.some-mixin(@newClass,@color,@color2){
      .@{newClass}{
         color: @color;
         & when not (@color = @color2){
            &:extend(.common-padding-diff-color);
        }
      }
}

.some-mixin(abc,#ffffff,#000000);
.some-mixin(xyz,#ffffff,#ffffff);
.some-mixin(jkl,#ff00f0,#ff00ff);

This would compile into

.common-padding-diff-color,
.abc,
.jkl {
  padding: 4px; /* style applied for all cases where colors are not same */
}
.abc {
  color: #ffffff;
}
.xyz {
  color: #ffffff;
}
.jkl {
  color: #ff00f0;
}