gistfile1.txt

float min_3d_length = FLT_MAX;
float max_3d_length = FLT_MIN;

for (set<vertex_3>::const_iterator i = final_vertices.begin(); i != final_vertices.end(); i++)
{
	if (i->length() > max_3d_length)
		max_3d_length = i->length();
		
	if (i->length() < min_3d_length)
		min_3d_length = i->length();
}


float max_rainbow = 360.0f;
float min_rainbow = 360.0f;

for (set<vertex_3>::const_iterator i = final_vertices.begin(); i != final_vertices.end(); i++)
{
	RGB rgb = HSBtoRGB(static_cast<unsigned short int>(
		max_rainbow - (((i->length() - min_3d_length) / (max_3d_length - min_3d_length)) * min_rainbow)),
		static_cast<unsigned char>(50),
		static_cast<unsigned char>(100));

	out << static_cast<float>(rgb.r) / 255.0f << " " << static_cast<float>(rgb.g) / 255.0f << " " << static_cast<float>(rgb.b) / 255.0f << " ";
}

// ... where HSBtoRGB is:

RGB HSBtoRGB(unsigned short int hue_degree, unsigned char sat_percent, unsigned char bri_percent)
{
	float R = 0.0f;
	float G = 0.0f;
	float B = 0.0f;

	if (hue_degree > 359)
		hue_degree = 359;

	if (sat_percent > 100)
		sat_percent = 100;

	if (bri_percent > 100)
		bri_percent = 100;

	float hue_pos = 6.0f - ((static_cast<float>(hue_degree) / 359.0f) * 6.0f);

	if (hue_pos >= 0.0f && hue_pos < 1.0f)
	{
		R = 255.0f;
		G = 0.0f;
		B = 255.0f * hue_pos;
	}
	else if (hue_pos >= 1.0f && hue_pos < 2.0f)
	{
		hue_pos -= 1.0f;

		R = 255.0f - (255.0f * hue_pos);
		G = 0.0f;
		B = 255.0f;
	}
	else if (hue_pos >= 2.0f && hue_pos < 3.0f)
	{
		hue_pos -= 2.0f;

		R = 0.0f;
		G = 255.0f * hue_pos;
		B = 255.0f;
	}
	else if (hue_pos >= 3.0f && hue_pos < 4.0f)
	{
		hue_pos -= 3.0f;

		R = 0.0f;
		G = 255.0f;
		B = 255.0f - (255.0f * hue_pos);
	}
	else if (hue_pos >= 4.0f && hue_pos < 5.0f)
	{
		hue_pos -= 4.0f;

		R = 255.0f * hue_pos;
		G = 255.0f;
		B = 0.0f;
	}
	else
	{
		hue_pos -= 5.0f;

		R = 255.0f;
		G = 255.0f - (255.0f * hue_pos);
		B = 0.0f;
	}

	if (100 != sat_percent)
	{
		if (0 == sat_percent)
		{
			R = 255.0f;
			G = 255.0f;
			B = 255.0f;
		}
		else
		{
			if (255.0f != R)
				R += ((255.0f - R) / 100.0f) * (100.0f - sat_percent);
			if (255.0f != G)
				G += ((255.0f - G) / 100.0f) * (100.0f - sat_percent);
			if (255.0f != B)
				B += ((255.0f - B) / 100.0f) * (100.0f - sat_percent);
		}
	}

	if (100 != bri_percent)
	{
		if (0 == bri_percent)
		{
			R = 0.0f;
			G = 0.0f;
			B = 0.0f;
		}
		else
		{
			if (0.0f != R)
				R *= static_cast<float>(bri_percent) / 100.0f;
			if (0.0f != G)
				G *= static_cast<float>(bri_percent) / 100.0f;
			if (0.0f != B)
				B *= static_cast<float>(bri_percent) / 100.0f;
		}
	}

	if (R < 0.0f)
		R = 0.0f;
	else if (R > 255.0f)
		R = 255.0f;

	if (G < 0.0f)
		G = 0.0f;
	else if (G > 255.0f)
		G = 255.0f;

	if (B < 0.0f)
		B = 0.0f;
	else if (B > 255.0f)
		B = 255.0f;

	RGB rgb;

	rgb.r = static_cast<unsigned char>(R);
	rgb.g = static_cast<unsigned char>(G);
	rgb.b = static_cast<unsigned char>(B);

	return rgb;
}

I'm not sure where I got the HSBtoRGB code from. :(