Create three-color gradient like Illustrator color picker

Question

Problem:

I am having difficulties figuring out how to create a three-color gradient (white, black and blue) like the one used in Illustrator's color picker (left color box).

Example #1 (how it should be): Example #2 (how it is now): Desired output:

To get the gradient in three colors with white on the top-left corner, black below, and blue in the top-right corner.

Answer 1

This is for Illustrator

1. Draw a white filled square

2. Use the Mesh tool (U) to select a corner

3. Double click on the fill colour icon, choose a colour from the colour picker

4. Repeat steps 2 and 3 for each corner

Example