PowerBI Tile on O365 Home Page

You can't change the color of one specific app tile. The preset icon colors represent the inidivual product branding.
 
For accessibility, you could change your browser to ignore the colours. It then displays the icons with no colored square and just a text label. It looks strange to me, but I can't find any other option to help you out. 
 
On the new app launcher, all my tiles are grey by default with a white text label until I hover over them. That's also a little better than the online app tiles on the home page.
 
Use accessibility features in your web browser
Because the Office 365 portal is a service that you view in a web browser, some accessibility features are supplied by the browser itself. WindowsInternet Explorer versions 8 through 11 offer features like zoom, customizable font sizes and colors, and compatibility with screen readers.
For example, you can make text easier to read in Internet Explorer by setting high-contrast view options.
In the Internet Explorer toolbar, to open the Tools menu, move the focus to the Tools button, and press the Spacebar.
Move the focus to Internet options, and press Enter.
On the General tab, which has the focus, move to the Accessibility button, and press the Spacebar.
In the Accessibility dialog box, select the check boxes labeled Ignore colors specified on webpages, Ignore font styles specified on webpages, and Ignore font sizes specified on webpages. Press Enter.
In the Internet Options dialog box, tab to the OK button, and press Enter.

-Sonia