Here's some CSS you can add to your form.  Then just add the CSS Class name (in this example, red, orange, yellow, green, blue, or purple) to the individual fields you want to have color background.

The input option will get most fields like single-line, currency, etc.

The select option is for drop-down fields.

The textarea option will get the multi-line fields.

.red input, .red select, .red textarea { 
  color : white;
  background-color : red;
  border-width : 1px; 
  border-color : #A9A9A9; 
} 

.blue input, .blue select, .blue textarea { 
  color : white;
  background-color : blue;
  border-width : 1px; 
  border-color : #A9A9A9; 
} 

.green input, .green select, .green textarea { 
  color : white;
  background-color : green;
  border-width : 1px; 
  border-color : #A9A9A9; 
} 

.orange input, .orange select, .orange textarea { 
  color : black;
  background-color : orange;
  border-width : 1px; 
  border-color : #A9A9A9; 
} 

.yellow input, .yellow select, .yellow textarea { 
  color : black;
  background-color : yellow;
  border-width : 1px; 
  border-color : #A9A9A9; 
} 

.purple input, .purple select, .purple textarea { 
  color : white;
  background-color : purple;
  border-width : 1px; 
  border-color : #A9A9A9; 
} 

To set-up different colors, just change the color names or use color codes (example: #FF0000 is red).

Here's a website with lots of color options: https://www.w3schools.com/colors/colors_names.asp