Add Values to a Sequence?

You have to create a new sequence by concatenating x and a sequence containing only the new element:

<#assign x = x + [ "green" ] />

FreeMarker is basically a write-once language. It tries very hard to make it impossible to manipulate data, and that includes modifying arrays or maps, etc.

You can work around this, however, through concatenation and reassignment:

<#assign my_array = [] />
<#list 1..10 as i>
  <#assign my_array = my_array + ["value " + i] />
</#list>

This should result in an array containing "value 1" through "value 10". If this seems inelegant it's because it was intended that way. From FreeMarker's ideological perspective, once you've started building arrays, etc., you've moved beyond what the templating language should be doing and into what the models, controllers, helper classes, etc., should be doing in Java code. Working in FreeMarker can become intensely frustrating the more you deviate from this viewpoint.

From http://freemarker.sourceforge.net/docs/app_faq.html#faq_modify_seq_and_map:

The FreeMarkes Template Language doesn't support the modification of sequences/hashes. It's for displaying already calculated things, not for calculating data. Keep templates simple. But don't give it up, you will see some advices and tricks bellow.