Can only chime in, you are putting effort into collaborative editing, but can not provide many of the basic features regarding table functions and formatting that are present in competitive platforms, what is that about?

To get wide acceptance for Confluence in an organisation, you also need to address the "common users" eg non-devs and they need easy and functional options in the editor, not a lot of nested macros, etc that makes the edit process hard to understand and easy to mess up.

We had a similar problem in Confluence 6.0.4
In our case it seems to be result of a "print" CSS

/* module-key = 'com.vertuna.confluence.plugins.confiforms:confiformResources', location = '/com/vertuna/confluence/plugins/confiforms/js/fullcalendar.print.css' */
/*!
 * FullCalendar v2.5.0 Print Stylesheet
 * Docs & License: http://fullcalendar.io/
 * (c) 2015 Adam Shaw
 */

/*
 * Include this stylesheet on your page to get a more printer-friendly calendar.
 * When including this stylesheet, use the media='print' attribute of the <link> tag.
 * Make sure to include this stylesheet IN ADDITION to the regular fullcalendar.css.
 */

.fc {
	max-width: 100% !important;
}


/* Global Event Restyling
--------------------------------------------------------------------------------------------------*/

.fc-event {
	background: #fff !important;
	color: #000 !important;
	page-break-inside: avoid;
}

.fc-event .fc-resizer {
	display: none;
}


/* Table & Day-Row Restyling
--------------------------------------------------------------------------------------------------*/

th,
td,
hr,
thead,
tbody,
.fc-row {
	border-color: #ccc !important;
	background: #fff !important;
}

As you can see this seems to be injected by the Confiforms Plugin 

I would like the ability to use macros to set table background color (more like fill), for the full height and width of the cell. bgcolor only highlights the text, not the full height and text of the table cell.

Where's the documentation on how to set even basic background cell colors? I don't see anything in the table editor that looks remotely applicable.

It is possible to set custom colors, if you're willing to edit the table cell definition in Atlassian's pseudo XHTML editor.

For example, this table:
{{<table>
<tbody>
<tr>
<td style="background-color: rgb(100,149,237);">
<p>
<strong>Has a Cornflower background</strong>
</p>
<p> </p>
</td>
</tr>
</tbody>
</table>
}}

Results in a one row, one cell table that looks like this:

Note: Color name generally does not work. You need to enter the color either in the decimal RGB form as shown, or in Hex (#rrggbb). If you enter it in hex, the Atlassian editor will convert it to the rgb decimal form.

This is undocumented behavior. Atlassian may break it in the future. This is documented to work on Confluence 5.4.4.

Fully agree with @Michael Brinson comments (from 2014!!!!!!!!)
So, Atlassian team, what about it?

Atlassian either needs to re-open this issue and add the votes from https://jira.atlassian.com/browse/CONF-27618 to this one, or else add all of the votes from this one over to https://jira.atlassian.com/browse/CONF-27618.

It's great that the ability to set background colors for cells has been added, but the description for this request is "There is no present way to set CUSTOM colours ..." Adding 5 set colors is NOT CUSTOM.

Vote up the latest issue ticket regarding this feature here: https://jira.atlassian.com/browse/CONF-27618

It's not possible in v5.6.1 either. There's no reason why we should not be able to set custom background colors in tables. There are options on the font colors, so why not the tables? On that topic, we should be able to set any hex color value for fonts and backgrounds via the editor. Why is this not available?

jan.mueller, it is possible, just very limited. There are five colors to choose from...
Look for the 'Highlight' button on the right side of the table toolbar. https://confluence.atlassian.com/display/DOC/Working+with+Tables