How do I select a specific product variant by element/field/entry?
You would currently need to loop over all variants and just grab the ones that match the 'if' condition shown below:

{% set colourSlug = craft.request.getSegment(4) %}

{% if not colourSlug %}
    {# Grab first product variant #}
    {% set dressColour = product.defaultVariant %}
{% else %}
    {# Set dress colour #}
    {% for variant in product.variants %}
    {% if variant.colour == colorSlug %}
       {{variant.sku}}
    {% endif %}
    {% endfor %}
{% endif %}


We will look to enable variants to be an element criteria query. If each variant has their own page, would it make sense they are seperate products?