Since this isn't in the product documentation, I wanted to share the following for anyone who has experienced any type of color rendering issues when using the Marketo Deliverability Design Tool to test the html of a Marketo email template (example attached). Copied below is some feedback I recently received from one the Marketo Support engineers. Hope you find this helpful.
"After further exploration, I found that the deliverability tool cannot accept the syntax used to make the template editable in the WSYIWYG Email. When the email is ready to be sent out, that syntax is processed by the email processor which then allows for the correct colors to come through before the email is sent out. The deliverability tool does not have this syntax processing and thus the color will look off since, ultimately, no color would exist. However, you may be able to copy the HTML of the email once sent as a sample to yourself and it should show up correctly as that email would already have been processed for general use and the html should have the correct colors in that form. "