Hi,
Please open file style.css and add the following code:

.menu-nav>li>a .fa {
color: #399;
}

In the above example, we use color: #399; you can replace with the one you want

Hi,
Do you mean you want to change the color of Previous Project and Next Project?
Please see screenshot here https://www.dropbox.com/s/c60b35n2mtxycfc/1.png?dl=0
Do you want change the black or blue (when hover)

Hi,
Please add the following code at the bottom of file style.css

.single-portfolio .owl-theme .owl-controls .owl-buttons div {
color: #399;
background-color: rgba(255,255,255,.3);
}

color: color of the arrows
background: background color