From the Dashboard page, go to Appearance on the left and select Customize from the list that pops up.
On the next page you should see your website in the right and a list of items you can change on the left. On the website side navigate to your contact page and scroll so you can see where the button should be.
On the left scroll down until you find CSS or Additional CSS or something similar and click on it.
Cut and paste the code below into this window and you should see the button turn grey. input{background-color:#cccccc !important;}
The part that says cccccc is actually the color grey. You can change this by picking another color and changing the cccccc to something like this 3399ff, which is light blue. See more colors here – https://www.w3schools.com/colors/colors_picker.asp
When you are done don’t forget to to click on the blue button at the top that says Publish to make it live.