See the example written in Java: http://www.pdflib.com/pcos-cookbook/pages/page-colors/

pdflib_pcos_colorspace.php

<?php
try {
    $pdflib = new PDFlib;
    $colorspace = new Colorspace($pdflib);
    print_r ($colorspace->analyze('test.pdf'));
} catch (PDFlibException $e) {
    print_r ($e);
}

class Colorspace
{
    protected $data = array();

    public function __construct(PDFlib $p)
    {
        $this->p = $p;
    }

    public function analyze($filename)
    {
        $this->data = array();

        $doc = $this->p->open_pdi_document(realpath($filename), '');

        $this->data['filename'] =  $this->p->pcos_get_string($doc, "filename");

        $pagecount = (int) $this->p->pcos_get_number($doc, "length:pages");

        for ($page = 0; $page < $pagecount; $page++) {
            $this->analyzeColorspaces($doc, $page);
        }

        return $this->data;
    }

    public function analyzeColorspaces($doc, $page)
    {
        $colorspacecount = (int) $this->p->pcos_get_number($doc, "length:pages[" . $page . "]/colorspaces");

        $page++;

        if ($colorspacecount > 0) {
            $this->data['pages'][$page]['colorspace_count'] = $colorspacecount;

            for ($i = 0; $i < $colorspacecount; $i++) {
                $this->data['pages'][$page]['colorspaces'][] = $this->analyzeColorspace($doc, $page, "pages[" . ($page - 1) . "]/colorspaces[" . $i . "]");
            }
        }
    }

    private function analyzeColorspace($doc, $page, $colorspace_path)
    {
        $name = $this->p->pcos_get_string($doc, $colorspace_path . "/name");
        $components = $this->p->pcos_get_number($doc, $colorspace_path . "/components");

        $colorspace = array(
            'name' => $name,
            'components' => $components
        );

        if ($name === "ICCBased") {
            $colorspace['icc_channels'] = $this->p->pcos_get_number($doc, $colorspace_path . "/csarray[1]/N");
            $colorspace['icc_profile_bytes'] = $this->p->pcos_get_number($doc, $colorspace_path . "/csarray[1]/Length");
        } elseif ($name === "Lab" || $name === "CalGray" || $name === "CalRGB") {
            for ($j = 0; $j < 3; $j += 1) {
                $colorspace['whitepoint'][] = $this->p->pcos_get_number($doc, $colorspace_path . "/csarray[1]/WhitePoint[" . $j . "]");
            }
        } elseif ($name === "Separation") {
            $colorspace['colorantname'] = $this->p->pcos_get_string($doc, $colorspace_path . "/colorantname");
            $alternateid = (int) $this->p->pcos_get_number($doc, $colorspace_path . "/alternateid");
            $colorspace['alternateid'] = $this->analyzeColorspace($doc, $page, "colorspaces[" . $alternateid . "]");
        } elseif ($name === "DeviceN") {
            $colorantcount = (int) $this->p->pcos_get_number($doc, "length:" . $colorspace_path . "/colorantnames");
            $colorspace['colorcount'] = $colorcount;
            for ($j = 0; $j < $colorantcount; $j += 1) {
                $colorspace['colorantnames'][] = $this->p->pcos_get_string($doc, $colorspace_path . "/colorantnames[" . $j . "]");
            }
            $alternateid = (int) $this->p->pcos_get_number($doc, $colorspace_path . "/alternateid");
            $colorspace['alternateid'] = $this->analyzeColorspace($doc, $page, "colorspaces[" . $alternateid . "]");
        } elseif ($name === "Indexed") {
            $palettesize = (int) $this->p->pcos_get_number($doc, $colorspace_path . "/csarray[2]");
            $colorspace['palettesize'] = $palettesize;
            $baseid = (int) $this->p->pcos_get_number($doc, $colorspace_path . "/baseid");
            $colorspace['baseid'] = $this->analyzeColorspace($doc, $page, "colorspaces[" . $baseid . "]");
        }

        return $colorspace;
    }
}