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Often I like to reduce the alpha value (level of transparency) of colours to identify patterns of over-plotting when displaying lots of data points with R. So, here is a tiny function that allows me to add an alpha value to a given vector of colours, e.g. a RColorBrewer palette, using Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.
col2rgb and rgb, which has an argument for alpha, in combination with the wonderful apply and sapply functions. The example below illustrates how this function can be used with colours provided in different formats, thanks to the
col2rgb function. 
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