SASS fade_in effect problems to watch - css

Question

I am trying to get a differents divs with gradients color, from darker to lighter.
I can watch how my code edit lighter the color, but i can't watch the darker effect. I review the code and all are working correctly. The fade_out effect is working, but not the fade_in effect - at it is identical. When I review the css file generated, i watch that the background-color are the same that the $color-base value. Why the fade_in effect is not appreciating?
SCSS:
#if $k==1 {
$color-base: red;
}
#else if $k==2 {
$color-base: blue;
}
#else {
$color-base: green;
}
.container-#{$k} {
.base-color {
background-color: $color-base;
}
#for $i from 4 through 1 {
.darken-#{$i} {
background-color: fade_in($color-base, 0.2 * $i);
.color-box-text {
color: red;
}
}
.lighten-#{$i} {
background-color: fade_out($color-base, 0.2 * $i);
.color-box-text {
color: black;
}
}
}
}
}
HTML:
<div class="box darken-4"><span class="color-box-text">Darken 4</span></div>
<div class="box darken-3"><span class="color-box-text">Darken 3</span></div>
<div class="box darken-2"><span class="color-box-text">Darken 2</span></div>
<div class="box darken-1"><span class="color-box-text">Darken 1</span></div>
<div class="box base-color"><span class="color-box-text">Base color</span></div>
<div class="box lighten-1"><span class="color-box-text">Lighten 1</span></div>
<div class="box lighten-2"><span class="color-box-text">Lighten 2</span></div>
<div class="box lighten-3"><span class="color-box-text">Lighten 3</span></div>
<div class="box lighten-4"><span class="color-box-text">Lighten 4</span></div>
</div>
<div class="mycontainer container-2">
<div class="box darken-4"><span class="color-box-text">Darken 4</span></div>
<div class="box darken-3"><span class="color-box-text">Darken 3</span></div>
<div class="box darken-2"><span class="color-box-text">Darken 2</span></div>
<div class="box darken-1"><span class="color-box-text">Darken 1</span></div>
<div class="box base-color"><span class="color-box-text">Base color</span></div>
<div class="box lighten-1"><span class="color-box-text">Lighten 1</span></div>
<div class="box lighten-2"><span class="color-box-text">Lighten 2</span></div>
<div class="box lighten-3"><span class="color-box-text">Lighten 3</span></div>
<div class="box lighten-4"><span class="color-box-text">Lighten 4</span></div>
</div>
<div class="mycontainer container-3">
<div class="box darken-4"><span class="color-box-text">Darken 4</span></div>
<div class="box darken-3"><span class="color-box-text">Darken 3</span></div>
<div class="box darken-2"><span class="color-box-text">Darken 2</span></div>
<div class="box darken-1"><span class="color-box-text">Darken 1</span></div>
<div class="box base-color"><span class="color-box-text">Base color</span></div>
<div class="box lighten-1"><span class="color-box-text">Lighten 1</span></div>
<div class="box lighten-2"><span class="color-box-text">Lighten 2</span></div>
<div class="box lighten-3"><span class="color-box-text">Lighten 3</span></div>
<div class="box lighten-4"><span class="color-box-text">Lighten 4</span></div>
</div>

Answer 1

The function fade-in() is doing the same as opacify() and according to the documentation:
The opacify() function increases the alpha channel by a fixed amount.
Your $color-base values are all opaque and therefore the fade-in() function doesn't apply since their alpha channels have already the max value. If you try with a base color with an alpha value lower than 1 you will see that your code works fine.