How to replace characters in string How do I replace ";" with "," in string "Red; Green; Blue"?

{% set rgb = "Red; Green; Blue" %}
Twig has a replace filter for that. You can replace strings/vars with strings/vars: 

{% set string = "I love apples" %}
{% set var = "like" %}
{% set string = string|replace({'apples': 'kiwis', 'love': var}) %}

{{ string }} {# Prints "I like kiwis" #}


Your example:

{% set rgb = "Red; Green; Blue" %}
{% set rgb = rgb|replace({';': ','}) %}

{{ rgb }} {# Prints "Red, Green, Blue" #}