How can i style “echo apply_filters”

apply_filters just calls any methods hooked to the filter, in this case woocommerce_attribute, and returns the output. You could do a couple of things to style this, here’s one way:

    $filtered = apply_filters(
        'woocommerce_attribute',
        wpautop( wptexturize( implode( ', ', $values ) ) ),
        $attribute,
        $values
    );

    echo '<span class="attribute-' . esc_attr( $attribute ) . ">' . $filtered . '</span>';

This will create something like:

<span class="attribute-color">RSV</span>

Now, you can style this in CSS with a rule such as:

span.attribute-color {
    font-weight: bold;
    /* etc */
}

This is assuming the attribute’s name is “color”, but whatever it is you should now be able to target that particular bit of text and apply custom CSS to it.