\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{fadings}
\begin{document}
\begin{tikzpicture}
\fill[black] (90:4) -- (210:4) -- (-30:4) -- cycle;
\fill[green,transform canvas={rotate=120},path fading=south] (90:4) -- (210:4) -- (-30:4) -- cycle;
\fill[blue,transform canvas={rotate=-120},path fading=south] (90:4) -- (210:4) -- (-30:4) -- cycle;
\fill[red,path fading=south] (90:4) -- (210:4) -- (-30:4) -- cycle;
% avoid black border due to incorrect rendering
\draw[white,ultra thin] (90:4) -- (210:4) -- (-30:4) -- cycle;