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If you have overlapping features (I noticed with points and only tested with points), their colors will "stack" even when there is no blending chosen.

I have attached an image and a shapefile. The shapefile has 5 groups of overlapping points. The left most group has 16 points stacked on top of each other (copy'n'pasted), the next one 8, then 4 and 2. The right most is just 1 point feature.

In the image the top row is styled with a single symbol using the color "226, 255, 94" (RGB, chosen randomly as bright color). The graduated and the categorized rows both use the Spectral scheme from Color Brewer on the "id" column (which is '1' for all features). The points are 0.001 map units big and have no border.

I chose a black background, but it is visible on white just as well. I used no blending options. It does not matter if the "less jagged lines" option in rendering options is ticked or not, I saw no difference. I have not tested various outputs from the composer. This is when looking at the normal map canvas.

In the image you can see that the more points are stacked on top of each other, the brighter the spot appears in the canvas. Instead it should be using the color of whatever feature is on "top" by whatever metric QGIS uses for that. If all points are styled with the same color, there should be no visible difference between the groups, regardless of the amount of "underlying" point features.

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QGIS version 2.3.0-Master QGIS code revision dcbf893
Compiled against Qt 4.8.6 Running against Qt 4.8.6
Compiled against GDAL/OGR 1.11.0 Running against GDAL/OGR 1.11.0
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PostgreSQL Client Version 9.3.4 SpatiaLite Version 4.1.1
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QScintilla2 Version 2.8.1