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see the yellow??
Studio pictures of D65 n°5.
This triangle covers the paper with 40% of paint. In the middle, the average density of the 3 colors is the same : 40% divided by 3.
If you click on the picture to zoom, you will see that the red lines were made first, and parallel. The green lines are in-between the red and the blue ones. They are not parallel, and on the right side they are thinner than on the left side. This is how I solved the problem of superimposing 3 gradients over each other: 2 are non-linear (the red one is compressed longitudinally, the green one is linear on one side and compressed longitudinally on the other,) the blue one on the top is linear, i.e., the density varies between 40% and 0% proportionally to the distance between the beginning and the end (the corner and the opposite side.)
Close to the Green end is a neutral grey point, which is re-drawn in the square on the top right. The Blue end is the darkest, and the Green one the lightest. A line has been placed to show the direction of equi-luminance.