Changing the product old price color
The old price is the crossed out price that can optionally be shown before the real price. It can be set at the Advanced tab in product management. This scripts allows you to personalize the color of the old price using CSS. 

These are the steps to add personalized CSS code to the design:

1. Navigate to Settings > Layout > Designer and launch the designer
2. Click Preferences (last icon) and enable Advanced
3. Click Customize CSS (penultimate icon)
4. Add the piece of CSS code at the bottom (copy and paste)

Publish the design (first icon in the designer window) to make sure the result is live.

This is the CSS code to adjust the color of the old price:

.webshop_old_price { color: #c00; }

The color codes (such as #c00 for dark red) can be customized.

The W3Schools website has a "color picker" for easy color selection:
https://www.w3schools.com/colors/colors_picker.asp

Learn more about HTML/CSS color coding:
https://www.w3schools.com/colors

Color Designer - easily generate color palettes
https://colordesigner.io

Color Combos - create color combinations
https://www.colorcombos.com