While googling the other day, I ran across this old post (2017) that asked how to set the Font Color of a character style back to "No Color". https://answers.microsoft.com/en-us/msoffice/forum/all/how-can-i-set-a-character-style-colour-to-no-color/8ff4f7c3-063b-4df7-b4b8-6bac96dbbaf3
Maybe there's a newer post showing a different way, but I didn't run across any. So, I thought I'd show a way that doesn't require creating a new document, recreating the character style, and copying the style back to the original document. Perhaps, some of you may already know this method, but for the ones that don't, here is how it's done. This works with the Windows desktop version of Word. I don't know if it works with other versions.
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As a result of discussions in this thread with Charles, I added an image and updated the 4 steps to provide further clarification. To maintain the history of this thread, I applied strikethrough to the original steps, rather than deleting them.
Let's assume you have a character style named "Style 1". To change its Font Color to "No Color", perform the following steps:
1. Create a new paragraph in the document using the Normal style, and make a note of the Font Color the Normal style is using (e.g., Automatic, Red, Blue, etc.).
2. Apply the character style to some text in the paragraph.
3. With that text still selected, change its font color to the same color you noted in step 2, but do this by clicking on the Font Color icon in the Font group on the ribbon.
4. With the text still selected, right click on the character style in the quick style gallery, and choose "Modify Style 1 to Match Selection".
NOTE: Except for step 4, when the phrase 'Font Color' is used, the phrase is referring to the color displayed in the Font Color dropdown box shown in a style's Font dialog box.

Let's assume you have a character style named "Style 1", and its Font Color has been assigned to a specific color (e.g., red), and you want to change it to "No Color". First, be sure Style 1 is shown in the quick styles gallery. To change Style 1's Font Color to "No Color", perform the following steps:
- Create a new paragraph in the document using the Normal style. You can use any paragraph style, but I chose the Normal style in these steps.
- Make a note of the Font Color the Normal style is using (e.g., Automatic, Red, Blue, etc.).
- Apply the Style 1 character style to some text in the paragraph.
- With that text still selected, change its color to the same color you noted in step 2, but this time do it by clicking on the Font Color icon in the Font group on the ribbon.
- With the text still selected, right click on Style 1 in the quick style gallery, and choose "Modify Style 1 to Match Selection".
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Now, when you right click on Style 1 in the quick style gallery, and you choose Modify > Format > Font, you'll see the Font Color set to "No Color". If you want to change the Underline Color to "No Color", the process is similar. Surprisingly, modifying Style 1 directly and setting its Font Color to the same color as the Normal style doesn't appear to work.