Disclaimer: You require OptiFine for this.
In your resource pack, add a new folder called "optifine" (without quotes) in assets/minecraft. Open up the folder called optifine and add a new text file. Make sure "Show file extensions" is on. Then rename the text file to "color.properties". A popup should show up asking if you would like to change the file extension. Click "Yes" and continue.
Now, open color.properties and add the following string:
particle.portal=(desired hex color)
Hope this helps!
