The icon color for Kiwi's Size Chart link and the Size Recommender button are controlled separately. Follow the steps below depending on which one you want to update.
Changing the Size Chart Icon Color
In your Shopify admin, go to Apps > Kiwi Size Chart & Recommender.
Open Styles & Settings.
Select the General Settings tab.
Under General Settings, click the Modal sub-tab.
In the Modal Settings section, find Icon Color next to the Link icon field.
Click the color swatch and choose your new color.
Check the Size Chart Preview panel on the right to confirm the new color appears as expected.
Click Save Settings.
Changing the Size Recommender Icon Color
In your Shopify admin, go to Apps > Kiwi Size Chart & Recommender.
Open Styles & Settings.
Select the Recommender tab.
Click the Display sub-tab.
Scroll down to Injected Button Settings & Styles.
Check the Icon Color checkbox if it isn't already enabled, then click the color swatch to pick your new color.
Review the Recommender Button Preview panel on the right to confirm the change.
Click Save Settings.
Note: These two icon color settings are independent. If you're using both the Size Chart link and the Recommender button on your product pages, you'll need to update each one separately to keep them visually consistent.