After snooping around the Ruby source and looking how it's done there, it looks like this is the way to go:
<span class="syntaxdefault">struct Color<br /></span><span class="syntaxkeyword">{<br /></span><span class="syntaxdefault"> unsigned char red</span><span class="syntaxkeyword">,</span><span class="syntaxdefault"> green</span><span class="syntaxkeyword">,</span><span class="syntaxdefault"> blue</span><span class="syntaxkeyword">,</span><span class="syntaxdefault"> alpha</span><span class="syntaxkeyword">;<br />};<br /><br /></span><span class="syntaxdefault">static VALUE<br />color_to_sketchup</span><span class="syntaxkeyword">(</span><span class="syntaxdefault"> Color color </span><span class="syntaxkeyword">)<br />{<br /></span><span class="syntaxdefault"> VALUE mSketchup</span><span class="syntaxkeyword">,</span><span class="syntaxdefault"> cColor</span><span class="syntaxkeyword">;<br /></span><span class="syntaxdefault"> VALUE skp_color</span><span class="syntaxkeyword">,</span><span class="syntaxdefault"> args</span><span class="syntaxkeyword">[</span><span class="syntaxdefault">4</span><span class="syntaxkeyword">];<br /></span><span class="syntaxdefault"> <br /> mSketchup </span><span class="syntaxkeyword">=</span><span class="syntaxdefault"> rb_const_get</span><span class="syntaxkeyword">(</span><span class="syntaxdefault"> rb_cObject</span><span class="syntaxkeyword">,</span><span class="syntaxdefault"> rb_intern</span><span class="syntaxkeyword">(</span><span class="syntaxdefault"> </span><span class="syntaxstring">"Sketchup"</span><span class="syntaxdefault"> </span><span class="syntaxkeyword">)</span><span class="syntaxdefault"> </span><span class="syntaxkeyword">);<br /></span><span class="syntaxdefault"> cColor </span><span class="syntaxkeyword">=</span><span class="syntaxdefault"> rb_const_get_at</span><span class="syntaxkeyword">(</span><span class="syntaxdefault"> mSketchup</span><span class="syntaxkeyword">,</span><span class="syntaxdefault"> rb_intern</span><span class="syntaxkeyword">(</span><span class="syntaxstring">"Color"</span><span class="syntaxkeyword">)</span><span class="syntaxdefault"> </span><span class="syntaxkeyword">);<br /></span><span class="syntaxdefault"> <br /> args</span><span class="syntaxkeyword">[</span><span class="syntaxdefault">0</span><span class="syntaxkeyword">]</span><span class="syntaxdefault"> </span><span class="syntaxkeyword">=</span><span class="syntaxdefault"> rb_float_new</span><span class="syntaxkeyword">(</span><span class="syntaxdefault"> color</span><span class="syntaxkeyword">.</span><span class="syntaxdefault">red </span><span class="syntaxkeyword">);<br /></span><span class="syntaxdefault"> args</span><span class="syntaxkeyword">[</span><span class="syntaxdefault">1</span><span class="syntaxkeyword">]</span><span class="syntaxdefault"> </span><span class="syntaxkeyword">=</span><span class="syntaxdefault"> rb_float_new</span><span class="syntaxkeyword">(</span><span class="syntaxdefault"> color</span><span class="syntaxkeyword">.</span><span class="syntaxdefault">green </span><span class="syntaxkeyword">);<br /></span><span class="syntaxdefault"> args</span><span class="syntaxkeyword">[</span><span class="syntaxdefault">2</span><span class="syntaxkeyword">]</span><span class="syntaxdefault"> </span><span class="syntaxkeyword">=</span><span class="syntaxdefault"> rb_float_new</span><span class="syntaxkeyword">(</span><span class="syntaxdefault"> color</span><span class="syntaxkeyword">.</span><span class="syntaxdefault">blue </span><span class="syntaxkeyword">);<br /></span><span class="syntaxdefault"> args</span><span class="syntaxkeyword">[</span><span class="syntaxdefault">3</span><span class="syntaxkeyword">]</span><span class="syntaxdefault"> </span><span class="syntaxkeyword">=</span><span class="syntaxdefault"> rb_float_new</span><span class="syntaxkeyword">(</span><span class="syntaxdefault"> color</span><span class="syntaxkeyword">.</span><span class="syntaxdefault">alpha </span><span class="syntaxkeyword">);<br /></span><span class="syntaxdefault"> <br /> skp_color </span><span class="syntaxkeyword">=</span><span class="syntaxdefault"> rb_class_new_instance</span><span class="syntaxkeyword">(</span><span class="syntaxdefault"> 4</span><span class="syntaxkeyword">,</span><span class="syntaxdefault"> args</span><span class="syntaxkeyword">,</span><span class="syntaxdefault"> cColor </span><span class="syntaxkeyword">);<br /></span><span class="syntaxdefault"> <br /> return skp_color</span><span class="syntaxkeyword">;<br />}</span><span class="syntaxdefault"> </span>