Description:
Bold colors, especially the lighter ones (yellow and green), are barely readable on a light background display. Thus don't use bold there, except for the main black forground color.

This can easily be accomplished in /usr/lib/yaourt/color.sh by selecting the yaourt_colors set depending on the value of USECOLOR or COLORMODE, e.g.

if ((USECOLOR==2)) || [[ $COLORMODE = "lightbg" ]]
then
local yaourt_colors="BOLD=1:BLINK=5:RED=31:GREEN=32:YELLOW=33:BLUE=34:PURPLE=35:CYAN=36"
else
local yaourt_colors="BOLD=1:BLINK=5:RED=1;31:GREEN=1;32:YELLOW=1;33:BLUE=1;34:PURPLE=1;35:CYAN=1;36"
fi

Likewise the pq_colors in the light background case should be adapted to

local pq_colors="no=0:other=0;35:testing=0;31:core=0;31:extra=0;32:local=0;33:nb=7;35:pkg=1:installed=0;35;7:votes=0;33;7:od=0;31;4:ver=0;34:lver=0;35;7:grp=0;34:dsc=0;34"

(Currently I do so by exporting YAOURT_COLORS in my ~/.bashrc.)