1. Create new skin
Please chack the guide: How to add new skin
You will need to change the skin around the one store view.
2. Change color in the new skin
Please find the file :
[MAGENTO_DIR] / skin / frontend / default / [YOUR_NEW_SKIN] / css / webandpeople / stylishstoreswitcher / stylishstoreswitcher.css
or
[MAGENTO_DIR] / skin / frontend / default / [YOUR_NEW_SKIN] / css / color.css/
Active tab styles:
Feel free to make changes.
This is a guide on how to set different tab colors for each Magento store in the Stylish Store Switcher Magento extension. This useful Magento extension displays stylish tabs instead of a select drop down.