How do I select a specific product variant by element/field/entry? Excuse my new-ness, but I'm having an issue with product variants. 

Scenario

I have a product with multiple variants (up to 30 in fact). Based on the URL, I want to choose a specific variant so that I can display fields from that variant on my product detail page. I also want a 'default variant' if there is no variant specified in the URL. 

In my example, the variant should be selected by a custom field in that variant colour whose value match the 4th URL segment.

Code

{# Grab colour segment #}
{% set colourSlug = craft.request.getSegment(4) %}

{# If there is no colour, we grab the first variant colour #}
    {% if not colourSlug %}
        {# Grab first product variant #}
        {% set dressColour = product.defaultVariant %}
    {% else %}
        {# Set dress colour #}
        ??{% set dressColour = product.variants.find(colour.colourSlug) %}??
    {% endif %}
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?