This demo uses OpenGL to draw a triangle. The vertices of the triangle can be different colors, and the colors of pixels in the interior are obtained by interpolating the colors from the vertices. The sliders give you some control over the vertex colors. The sliders set the hues for the colors in the HSV color space; the saturation and brightness of the colors are fixed at 1.
Hidden feature: Double-click the triangle to set the vertex colors to yellow, cyan, and magenta. Shift-double-click to set the colors to red, green, and blue.
Notice that when the vertices are red, green, and blue, the center of the triangle is gray, not white as you might expect. This is because the interpolation is in the RGB color space, and the values for R, G, and B at the center will all be about 0.5.