Why the combination of yellow and blue is gray in TikZ?

In page 1003/1318 TikZ manual there is an example to generate the array of colorful balls as below:

The ball color in (x=3, y=2) is made by combining 50% yellow and 50% blue according to the 2 variable for loop. But instead of green it is gray. Do you know why?
Below is my code:

documentclass{standalone}
usepackage{tikz}
begin{document}

begin{tikzpicture}[shading=ball]
foreach x / cola in {0/red,1/green,2/blue,3/yellow}
foreach y / colb in {0/red,1/green,2/blue,3/yellow}
shade[ball color=cola!50!colb] (x,y) circle (0.4cm);
end{tikzpicture}
end{document}