How to add css class to a field from a select list before rendering?
I have used the Field Formatter CSS class module for the same task. The client wanted to have predefined colors for heading elements in paragraph bundles. I have to admit this seems like an amendable approach, but it worked.

I had set up a vocabulary for the color names list and referenced this in the paragraph bundle as a select list (taxonomy term reference). In the display settings for the field is css-class selected. The format settings are set to target tag = entity. 

This results basically in following markup:

&lt;div class="entity entity-paragraphs-item paragraphs-item h1-colorclass-red"&gt;
    &lt;div class="field-name-field-heading"&gt;This is a headline&lt;/div&gt;
&lt;/div&gt;


CSS:

.h1-colorclass-red .field-name-field-heading {
    color: red;
}