Function to add transparent (alpha) into colors
visColoralpha is supposed to add transparent (alpha) into
input colors. It can be vector of R color specifications, such as a color name (as listed by 'colors()), a hexadecimal string of the form "#rrggbb" or "#rrggbbaa"
numeric vector of values in the range [0, 1] for alpha transparency channel (0 means transparent and 1 means opaque)
a vector of colors (after transparent being added)
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