A couple ways this can be done:
This will make it a square
ul
{
list-style-type: square;
}
This will make it green
li
{
color: #0F0;
}
This will prevent the text from being green
li p
{
color: #000;
}
However that will require that all text within lists be in paragraphs so that the color is not overridden.
A better way is to make an image of a green square and use:
ul
{
list-style: url(green-square.png);
}