Change calendar event background color in SharePoint 2013
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This SharePoint 2013 tutorial explains, how to change calendar event background color using CSS in SharePoint 2013.
By default when you create an event in SharePoint 2013/Online calendar, the background color will appear in blue like below:

Change calendar event background color in SharePoint 2013
You can change this background color using CSS. Edit the calendar page and add a script editor web part. To add a script editor web part, edit the page then from the Ribbon click on INSERT -> Web Part -> then from the categories select Media and Content and then choose Script editor web part.
<style type=”text/css”>
.ms-acal-item {
BORDER-BOTTOM: #ed0033; BORDER-LEFT: #ed0033; BACKGROUND-COLOR: #ed0033; BORDER-TOP: #ed0033; BORDER-RIGHT: #ed0033
}
.ms-acal-default-hover {
BORDER-BOTTOM: #ed0033; BORDER-LEFT: #ed0033; BACKGROUND-COLOR: #ed0033; BORDER-TOP: #ed0033; BORDER-RIGHT: #ed0033
}
.ms-acal-apanel-item {
BORDER-BOTTOM: #00485b; BORDER-LEFT: #ed0033; BACKGROUND-COLOR: #ed0033; BORDER-TOP: #ed0033; BORDER-RIGHT: #ed0033
}</style>
Then Save the page. Now the background color will appear like below:

Change calendar event background color in SharePoint 2016
You may like following SharePoint Calendar tutorials:
- Hide All Day Event Recurrence fields from a calendar in SharePoint 2013
- Create Calendar Overlays Using Single Calendar App in SharePoint 2013 Online
- Create Calendar Overlays Using Multiple Calendar Apps in SharePoint 2013 Online
- Create Color coded calendar in SharePoint 2013
- SharePoint 2013 Online Remove Time from All Events Calendar view
- SharePoint Online Add Calendar Month Week and Day view as Hyperlink
- Display the Calendar Using Jquery Full Calendar and SharePoint Rest API
- Working with SharePoint Calendar in SharePoint 2016/2013/Online
- SharePoint Online Group Calendar Web Part Tutorial
Hope this will be helpful to change SharePoint 2013 calendar background color in SharePoint 2013/2016 Or SharePoint Online.

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