The following piece of code will set the background color of the entire row:
function onFormSubmit(e) {
// get data and make it a 1D array
var time = e.range.getValues()[0];
// get the minutes and hours of the timestamp (time[0])
var minutes = time[0].getMinutes(), hours = time[0].getHours();
// calculate the total amount of minutes
var totalMin = (hours * 60) + minutes;
// do the logic (thanks to geekzspot for the 1087)
if(totalMin > 1087) {
e.range.setBackgroundColor('yellow');
} else {
e.range.setBackgroundColor('red');
}
}
UPDATE
This code will set only the back ground color of the timestamp:
function onFormSubmit(e) {
// get active spreadsheet and sheet
var ss = SpreadsheetApp.getActiveSpreadsheet();
var sh = ss.getActiveSheet();
// get data and make it a 1D array
var time = e.range.getValues()[0];
// get row index
var row = e.range.getRowIndex();
// get the minutes and hours
var minutes = time[0].getMinutes(), hours = time[0].getHours();
// calculate the total amount of minutes
var totalMin = (hours * 60) + minutes;
// set range to timestamp
var range = sh.setActiveCell("A"+row);
// do the logic (thanks to geekzspot for the 1087)
if(totalMin > 1087) {
range.setBackgroundColor('yellow');
} else {
range.setBackgroundColor('red');
}
}
Add this script, via the script editor, to your spreadsheet and set the trigger:

Here's the form: Test For Web Applications and here's the sheet