It may have something to do with the fact that qty_available and virtual_available are function fields that are not stored (with a good reason) and does not have fnct_search implemented (due to complexity).  Try using other fields and see if it works.

Update your system. See one of the latest commits: https://github.com/odoo/odoo/commit/e28e01a70f397f9ded4bac1ff060b93bf6072e40

Thanks Ivan, but that code is the standard Odoo code - not mine. However, if I modify the code as below, it does work, which means that it's possibaly not because virtual_available is a function ? I think it has to do with the { }

<record id="view_stock_product_template_tree" model="ir.ui.view">
            <field name="name">product.template.stock.tree.inherit</field>
            <field name="model">product.template</field>
            <field name="inherit_id" ref="product.product_template_tree_view"/>
            <field name="arch" type="xml">
                <tree position="attributes">
                    <attribute name="colors">red:virtual_available&lt;=0</attribute>
            <!--         <attribute name="colors">{'red':virtual_available&lt;=0, 'blue':virtual_available&gt;=0 and state in ('draft', 'end', 'obsolete'), 'black':virtual_available&gt;=0 and state not in ('draft', 'end', 'obsolete')}</attribute>-->
                </tree> 
                <field name="uom_id" position="after">
                    <field name="qty_available"/>
                    <field name="virtual_available"/>
                </field>
                
                <field name="state" position="replace"/>
            </field>
        </record>