If you are having trouble with seeing the crosshair when you look into the sky or at a specific color background, what you can do is change your mouse1 button or your default fire button to not only have the fire command but also the change crosshair color command.
*edit have you tried the first 2 links of google?
http://www.chipchapin.com/Half-Life/crosshairs.php3
http://www.ticclan.com/tutorials/crossh ... rites.html
i don't know what the fire (default mouse1) command is for shooting, but let's say it is fire for now
so instead of binding your mouse1 to fire, bind it to fire, wait, adjust crosshair
it should go something like this bind mouse1 "fire, wait, adjust crosshair"
it should cycle through the colors each time you click.