Create a indexed list

It needs to be done something like the following

Merge[Association /@ {"white" -> 1, "white" -> 0, "blue" -> 2, 
   "green" -> 1, "green" -> 2, "yellow" -> 3}, Join]
%["white"]

{1, 0}

It must be noted that Suba has a great approach, and shows the proper usage of Merge in their answer.

However, if can you make the Association applied to a List, or use a List in any way, there are approaches that I consider to be much simpler which I will show below.

Let's start with the messier version:

Extract[Transpose@{Range@Length@#}][#]&@Inactivate@Association["white" -> 1,
      "white" -> 0,
      "blue" -> 2,
      "green" -> 1,
      "green" -> 2,
      "yellow" -> 3]//GroupBy[#,Extract[1]->Extract[2]]&


(* <|"white" -> {1, 0}, "blue" -> {2}, "green" -> {1, 2}, 
 "yellow" -> {3}|> *)

However, the syntax begins to simplify if there is a list contained within the Association:

Extract[{1,1}]@Defer@Association[{"white" -> 1,
      "white" -> 0,
      "blue" -> 2,
      "green" -> 1,
      "green" -> 2,
      "yellow" -> 3}]//GroupBy[#,Extract[1]->Extract[2]]&


(* same as above *)

Finally, we can realize the cleanest looking implementation of this method, if we are able to use a List of the Associations:

GroupBy[
      {"white" -> 1,
      "white" -> 0,
      "blue" -> 2,
      "green" -> 1,
      "green" -> 2,
      "yellow" -> 3},Extract[1]->Extract[2]]


(* same as above *)

I hope this explains a possible set of alternatives. If you already defined series and wish to use that, unfortunately I cannot currently find a good workaround for that.

Please, let me know if you have any questions!