How do I select a specific product variant by element/field/entry?

Question

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 %}

Answer 1

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?

Answer 2

Just before I saw luke's answer I wrote this:

{# 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 dressVariantActive = product.defaultVariant %}
    {% else %}
        {# Set outside the loop #}
        {% set dressVariantActive = '' %}

        {% for dressVariantSelected in product.variants %}
            {% if dressVariantSelected.colour.value == colourSlug %}
                {% set dressVariantActive = dressVariantSelected %}
            {% endif %}
        {% endfor %}
        {% if not dressVariantActive %}{% exit 404 %}{% endif %}
    {% endif %}

I wanted to include this as I found out a useful thing: Defining a variable inside a for loop is only possible if you set the variable before the loop.