I plan to write a painting program based on linear combinations of xy plane points (0,1), (1,0 ) and (0,0). Such system works identically to RGB, except that the primaries are not within the gamut but at the corners of a triangle that encloses the entire gamut, therefore allowing for all colors to be reproduced. I have seen the three points being referred to as X, Y and Z (upper case) somewhere, but I cannot find the page anymore.

My pixel format stores the intensity of each of those three components the same way as RGB does, together with alpha value. This allows using pretty much any image manipulation operation designed for RGBA without modifying the code.

What is my format called? Is it XYZA, RGBA or something else? Google doesn't seem to know of XYZA and RGBA will get confused with sRGB + alpha (which I also need to use in the same program).

Notice that the primaries X, Y and Z and their intensities have little to do with the x, y and z coordinates (lower case) that are more commonly used.