This tool helps you to change the default splash screen from Construct 2 for your HTML5 based games.
"You paid for a FULL version, no full version has a watermark"
First of all, rename your c2runtime.js file to something else to keep as a backup.
Then, drag it and drop it in the file selection field.
Now all the settings will appear, fill the ones you want to change. For the images try to use the same resolution and ratio.
You need an hexadecimal number to change the colors, you can use google color picker.
When you're finished, just click the save button and save the file as c2runtime.js in the same folder. After you tested your game to make sure everything is alright, you may delete the backup.js
Another suggestion, put all your images and hex colors in a folder because I bet you'll need to use this tool more than once.