.project {
  @include span-columns(6);
  @include omega(odd);
  @include media(max-width 767px) {
    @include fill-parent
  }
  position: relative;
  margin-bottom: 1.5rem;
  background-color: #AEB6C1;
}

.project--past {
  @include fill-parent;
  text-align: center;
}

.project__title {
  @include position(absolute, 1rem null null 1rem);
  @include media(max-width 414px) {
    color: #313535;
    padding: 0.4rem;
    background-color: rgba(245, 245, 245, 0.87);
    -webkit-box-shadow: 1px 1px 0px 0px rgba(0,0,0,0.14);
    -moz-box-shadow: 1px 1px 0px 0px rgba(0,0,0,0.14);
    box-shadow: 1px 1px 0px 0px rgba(0,0,0,0.14);
  }
  color: #f3f3f3;
}

.project__img {
  display: block;
}

.project {
  @include span-columns(6);
  @include omega(odd);
  @include media(max-width 767px) {
    @include fill-parent
  }
  position: relative;
  margin-bottom: 1.5rem;
  background-color: #AEB6C1;
}

.project--past {
  @include fill-parent;
  text-align: center;
  background-color: none;
}

.project__title {
  @include position(absolute, 1rem null null 1rem);
  @include media(max-width 414px) {
    color: #313535;
    padding: 0.4rem;
    background-color: rgba(245, 245, 245, 0.87);
    -webkit-box-shadow: 1px 1px 0px 0px rgba(0,0,0,0.14);
    -moz-box-shadow: 1px 1px 0px 0px rgba(0,0,0,0.14);
    box-shadow: 1px 1px 0px 0px rgba(0,0,0,0.14);
  }
  color: #f3f3f3;
}

.project__img {
  display: block;
}

Change past project class background-color
Change past project class background-color
