With TikZ 3.0, there’s a new kind of transparency: you can use blend modes.
This short Venn diagram example shows how the screen blend mode can be used to create a clear visual effect with just a few lines of code.
Overall, there are 16 blend modes to choose from: normal, multiply, screen, overlay, darken, lighten, color dodge, color burn, hard light, soft light, difference, exclusion, hue, saturation, color, luminosity.
The code for this example was written by Paul Gaborit, published on TeX.SE and then on TeXample.net. This abstract is adapted from that on TeXample.
Paul Gaborit (via TeXample.net)