Hue[h,s,b,a] plots colors on a scale from 0 to 1 corresponding to red-yellow-green-blue-red. s, b, and a refer to saturation, brightness, and opacity.

f[ r_, θ_ ] := Module[ { pt, z },
     z = r Exp[ I θ ];
     pt = { Re @ z,Im @ z };
     { Hue[ θ/(2 Pi), 1 - r, 1 - r], Point @ pt } ];

pts = Table[ f[ r, θ ], { r , 0, 1, .01 }, { θ, 0, 2Pi, .01 } ];

Graphics[ pts, Background -> Black ]