You have to edit the qweb view "report_saleorder_document" and add t-if statements.   I have a template where I filter bom materials based on their product category.  So at the top of each table I put this:

<tbody>

<t t-if="o.move_lines">

<t t-foreach="o.bom_id.bom_line_ids" t-as="line">

<t t-if="line.product_id.product_tmpl_id.categ_id.id==4">

<tr>

<td> 

<span t-field="line.name"/>

</td>  etc...

In your case you will want to check the variant at the top of the loop over items on your sales order.

<t t-if="line.product_id.attribute_value_ids.name == 'blue'">  or if you know the id of that variant

<t t-if="line.product_id.attribute_value_ids.id==3">

 There is a lot of help for customizing reports - just Google odoo qweb custom reports.