You can achieve it through code. Obtain the back arrow drawable, modify its color with a filter, and set it as back button.
final Drawable upArrow = getResources().getDrawable(R.drawable.abc_ic_ab_back_mtrl_am_alpha);
upArrow.setColorFilter(getResources().getColor(R.color.grey), PorterDuff.Mode.SRC_ATOP);
getSupportActionBar().setHomeAsUpIndicator(upArrow);
Revision 1:
Starting from API 23 (Marshmallow) the drawable resource abc_ic_ab_back_mtrl_am_alpha
is changed to abc_ic_ab_back_material
.
EDIT:
You can use this code to achieve the results you want:
toolbar.getNavigationIcon().setColorFilter(getResources().getColor(R.color.blue_gray_15), PorterDuff.Mode.SRC_ATOP);