Algorithm for a diagonal non-linear gradient

I'm looking for an algorithm to generate the following image:

I'm only interested in the data shown in the upper left half (diagonal) of the square.

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I'm currently at a starting point where I have the following simple linear function:

double GetColorAt(double x, double y)
{
   return 1 - (x + y);
}

where x runs from left to right and y from top to bottom in the ranges of [0...1]. The return value is a color component for RGB channels in the range of [0...1]. It generates me this: