Change this code to the colour you want. It is located in the jquery.cslider in the css folder, you will need access to ftp, download it change the code and then upload again in the same folder. Worked for me as I have it grey right now.
.da-slider .mask {
position:absolute;
top:0;
left:0;
width:100%;
height:100%;
background: #CCCCCC;
opacity:0.85;
filter: alpha(opacity=85);
}