For cell editing you can use Ext.grid.plugin.CellEditing
grid plugin. This plugin have edit( editor, context, eOpts )
event which is fired after cell was edited.
In listener for this event, you can get information about edited record from context
parameter and make AJAX request to server for validation.
For set background color of the grid cell you can use:
// get element object from HTML code
var rowEl = Ext.get(context.row);
var tdElIdx = context.colIdx + 1;
// find element of edited cell
var celEl = rowEl.down('td:nth(' + tdElIdx + ')');
// set background color of the cell
celEl.setStyle('background-color','red');