A single header include for all coloring needs, can be used, easily, to create a simple rainbow effect, and convert HSV, and RGB, into hex for web design.

coloring.hpp

#pragma once
#include <Windows.h>

namespace coloring {
	template< typename T >
	std::string int_hex(T val) {
		if (static_cast<int>(val) == 0)
			return std::string("00");

		char hexval[3];
		sprintf(hexval, "%00x", static_cast<int>(val));
		hexval[2] = 0;

		return std::string(hexval);
	}

	std::string rgb_hex(float r, float g, float b) {
		return int_hex(r) + int_hex(g) + int_hex(b);
	}

	class RGB_C {
	public:
		float r, g, b;

		RGB_C() {
			r = 0.f;
			g = 0.f;
			b = 0.f;
		}

		RGB_C(float nr, float ng, float nb) {
			r = nr;
			g = ng;
			b = nb;
		}
	};

	class HSV {
	public:
		float h, s, v;

		HSV() {
			h = 0.f;
			s = 0.f;
			v = 0.f;
		}

		HSV(float nh, float ns, float nv) {
			h = nh;
			s = ns;
			v = nv;
		}
	};

	RGB_C hsv_rgb(float H, float S, float V) {
		H = H * 360.f; S = S * 100.f; V = V * 100.f;

		float r = 0.f, g = 0.f, b = 0.f;

		float h = H / 360;
		float s = S / 100;
		float v = V / 100;

		int i = floor(h * 6);
		float f = h * 6 - i;
		float p = v * (1 - s);
		float q = v * (1 - f * s);
		float t = v * (1 - (1 - f) * s);

		switch (i % 6) {
		case 0: r = v, g = t, b = p; break;
		case 1: r = q, g = v, b = p; break;
		case 2: r = p, g = v, b = t; break;
		case 3: r = p, g = q, b = v; break;
		case 4: r = t, g = p, b = v; break;
		case 5: r = v, g = p, b = q; break;
		}

		RGB_C color;
		color.r = floorf(r * 255.f);
		color.g = floorf(g * 255.f);
		color.b = floorf(b * 255.f);

		return color;
	}
};

// simple rainbow
int main() {
	float rainbow_r = 255.f, rainbow_g = 0.f, rainbow_b = 0.f;
	while (true) {
		for (float i = 0.f; i < 255.f; i += 2.f) {
			coloring::RGB_C rgb = coloring::hsv_rgb(i / 256, 0.6f, 1.f);
			rainbow_r = rgb.r;
			rainbow_g = rgb.g;
			rainbow_b = rgb.b;

			std::this_thread::sleep_for(std::chrono::milliseconds(25));
		}
	}
	
	return 0;
}