You will have to the the use the colorPalette expression syntax as in the example below - you can simply copy this XML row into an existing dashboard to see how it works - it's a dummy search that just creates a random time and when it's in the out of hours range it goes red.
<row>
<panel>
<table>
<title>Turning the Time column red if outside hours 18:00 to 06:00</title>
<search>
<query>| makeresults
| eval _time=now() - (random() % 86400)
| eval Date=strftime(_time, "%F"), Time=strftime(_time, "%T")
| eval EventCode=4624, Account_Name="user ".(random() % 10)
| table Date Time EventCode Account_Name</query>
<earliest>-24h@h</earliest>
<latest>now</latest>
<sampleRatio>1</sampleRatio>
</search>
<option name="count">100</option>
<option name="dataOverlayMode">none</option>
<option name="drilldown">none</option>
<option name="percentagesRow">false</option>
<option name="refresh.display">progressbar</option>
<option name="rowNumbers">false</option>
<option name="totalsRow">false</option>
<option name="wrap">true</option>
<format type="color" field="Time">
<colorPalette type="expression">if(tonumber(substr(value,1,2))>=18 OR tonumber(substr(value,1,2))<6, "#FF0000", "#FFFFFF")</colorPalette>
</format>
</table>
</panel>
</row>
My example was XML for use in a classic dashboard - so if you take the entire XML below and create a new dashboard and paste in this into the source.
<dashboard version="1.1">
<row>
<panel>
<table>
<title>Turning the Time column red if outside hours 18:00 to 06:00</title>
<search>
<query>
index="winlogs" host=* source="WinEventLog:Security" Eventcode=4624 Logon_Type=2 OR Logon_Type=7 NOT dest_nt_domain="Window Manager" NOT dest_nt_domain="Font Driver Host"
| sort_time
| convert ctime(_time) as timestamp
| table timestamp,EventCode,Logon_Type,Account_Name,RecordNumber,status
</query>
<earliest>-24h@h</earliest>
<latest>now</latest>
<sampleRatio>1</sampleRatio>
</search>
<option name="count">100</option>
<option name="dataOverlayMode">none</option>
<option name="drilldown">none</option>
<option name="percentagesRow">false</option>
<option name="refresh.display">progressbar</option>
<option name="rowNumbers">false</option>
<option name="totalsRow">false</option>
<option name="wrap">true</option>
<format type="color" field="timestamp">
<colorPalette type="expression">if(tonumber(substr(value,12,2))>=18 OR tonumber(substr(value,12,2))<6, "#FF0000", "#FFFFFF")</colorPalette>
</format>
</table>
</panel>
</row>
</dashboard>
This is what an XML dashboard looks like. You can see your search in the <search> section and the <format> section is what defines your colours and testing the time range.
That documentation for the format is here
https://docs.splunk.com/Documentation/Splunk/9.1.0/Viz/TableFormatsXML