Hey @csalvian thanks for following up. Here is how you can do it.
Click the Gear icon > Custom Form Styles.
1) To change the design on an existing template, click Edit.
2) Or, to start a new style, click New style on the upper left, and then select Invoice, Estimate, or Sales Receipt
3) Changes made on the Design and Content tabs will apply to both the invoice and the invoice email. e.g. color, turning on/off business name
4) From the Design tab, select Try other colors
5) Select the desired color
6) You will be able to see your new color applied to your email in your Send email preview when you send your next invoice
We are currently making an update to the preview on the Email tab so that you will be able to see the updates directly while making your changes