To change the text and the color of the Site Actions menu copy and paste the following code at the end of your master page:

<script language="JavaScript">
$(document).ready(function ()
{
    $("span.ms-siteactionsmenuinner > a.ms-menu-a").html("My Actions");
    $("span.ms-siteactionsmenuinner > a.ms-menu-a").css('color', 'white');
});
</script>

A quick and dirty way of customizing the Site Actions menu is to add a MenuItemTemplate element straight to the menu on your master page.

At first find the following snippet on the master page:

<span class="ms-siteactionsmenu" id="siteactiontd">
    <SharePoint:SiteActions runat="server"
        accesskey="<%$Resources:wss,tb_SiteActions_AK%>"
        id="SiteActionsMenuMain"
        PrefixHtml=""
        SuffixHtml=""
        MenuNotVisibleHtml="&nbsp;">
        <CustomTemplate>
            <SharePoint:FeatureMenuTemplate runat="server"
                FeatureScope="Site"
                Location="Microsoft.SharePoint.StandardMenu"
                GroupId="SiteActions"
                UseShortId="true">

Then, insert a custom MenuItemTemplate element inside the FeatureMenuTemplate container:

<SharePoint:FeatureMenuTemplate runat="server"
    FeatureScope="Site"
    Location="Microsoft.SharePoint.StandardMenu"
    GroupId="SiteActions"
    UseShortId="true">
    ... other menu item templates goes here ...
    <SharePoint:MenuItemTemplate runat="server" 
        id="MenuItem_MyAction"
        Text="My Action"
        Description="Do something when the menu item My Action is selected."
        ImageUrl="/_layouts/images/visualupgradehh.png"
        MenuGroupId="300"
        Sequence="305"
        UseShortId="true"
        ClientOnClickNavigateUrl="˜site/_layouts/MyLayoutPages/MyAction.aspx"
        PermissionsString="ManageSubwebs"
        PermissionMode="All" />

Knowing the id (SiteActionsMenuMain) and the structure of the Site Actions menu it is possible to reference MenuItemTemplate elements in the code-behind:

if (SiteActionsMenuMain.Controls.Count > 0 && SiteActionsMenuMain.Controls[0] != null)
{
    if (SiteActionsMenuMain.Controls[0].Controls.Count > 2)
    {
        FeatureMenuTemplate menu = (SiteActionsMenuMain.Controls[0].Controls[2] as FeatureMenuTemplate);
        if (SiteActionsMenuMain.Controls[0].Controls[2] is FeatureMenuTemplate)
        {
            MenuItemTemplate item = menu.FindControl("MenuItem_MyAction") as MenuItemTemplate;
            // Do something with the MenuItem_MyAction item, for example, hide it: item.Visible = false;
        }
    }
}