Hi Panos,
Thank you for using Syncfusion products.
Instead of using QueryCellInfo event for conditional formatting, you can use the GridConditionalFormatDescriptor. Please refer to the below code example and sample for reference.
Code Example :
Syncfusion.Windows.Forms.Grid.Grouping.GridConditionalFormatDescriptor gridConditionalFormatDescriptor1 = newSyncfusion.Windows.Forms.Grid.Grouping.GridConditionalFormatDescriptor();
gridConditionalFormatDescriptor1.Appearance.AnyRecordFieldCell.Interior = newSyncfusion.Drawing.BrushInfo(System.Drawing.Color.FromArgb(((int)(((byte)(255)))), ((int)(((byte)(191)))), ((int)(((byte)(52))))));
gridConditionalFormatDescriptor1.Appearance.AnyRecordFieldCell.TextColor = System.Drawing.Color.White;
gridConditionalFormatDescriptor1.Expression = "[CustomerID] LIKE \'A*\'";
gridConditionalFormatDescriptor1.Name = "ConditionalFormat 1";
Sample :
https://www.syncfusion.com/downloads/support/forum/121387/ze/CS-2110415607
Note :
If the issue is still not resolved, please replicate your issue in this sample and send it to us, so that it would help us provide a prompt solution.
Thanks,
AL.Solai.