There is a very easy way I found to change the color block -
Make sure the page you are working on is open on your screen
Click home in the upper left
Go to the Design Tab
Go to Site Styles
Either type Collage into the search box or scroll down to Image Block>Collage
Click on the color circle
Replace the HSL code with your own HSL code
To find your HSL code, you can use a site such as this one
Either choose your color or add your hex code in the Color Value box if you have it
Copy the HSL code from H to the end of the parenthesis