Hi. To me, this seems related to the date. Today or late -> red. In the future -> Black.

Cheers

Laurent answer is correct, to more explain it pls check this view definition

    <record model="ir.ui.view" id="view_account_asset_asset_tree">
        <field name="name">account.asset.asset.tree</field>
        <field name="model">account.asset.asset</field>
        <field name="field_parent">child_ids</field>
        <field name="arch" type="xml">
            <tree string="Assets" colors="blue:(state == 'draft');grey:(state == 'close')">
                <field name="name"/>
                <field name="category_id"/>
                <field name="purchase_date"/>
                <field name="partner_id"/>
                <field name="purchase_value"/>
                <field name="value_residual"/>
                <field name="currency_id" groups="base.group_multi_currency"/>
                <field name="company_id" groups="base.group_multi_company"/>
                <field name="state"/>
            </tree>
        </field>
    </record>

The 'colors' tag defines the color behavior of lines based on the line's state