you need to put it as $color and not /color on the play command
really all you need is to read carefully the help file
The -c switch forces mIRC to interpret lines as actual commands instead of plain text
in other words you can make the txt file with commands so that mirc reads em like it so that if you add the color it would read like it is
also:
The -n switch makes the play command use /notice instead of /msg
The -a switch makes /play use specified alias instead of /msg or /notice
and about the /write part...really if you think about it thats what youre telling it to do.../write [help] so it will over write it of course
PS:"/help " is your best friend