I'm sorry if I have misunderstood the question of argv0.
I have entrered the transparent color via the dialog in adresse book tab colors then click on "add" button. You see on the left and on the top a combo box with available colors.
You'll see the 15 colors and a 16th color with lines which is the signification of a transparent color. the number 99 is seen after viewing the mirc.ini file.
(I know it is bad to write in it, but thanks to say it
I don't know how long the transparent color is installed on mIRC. So, is it a bug, or a lapse of memory of programmers or a setting that result this problem ?
In mirc.hlp, title page "control codes" there this line :
You can use color 99 to indicate a transparent color.