I've created a class with a label with forecolor red. I've put this label on a form. Then I change the forecolor of the label in the class to yellow. But the label on the form stays red. Is this normal behaviour ?

I've experienced this type of behaviour before, and in the past insuring that the form redrew itself helped.
you can try frmName.ActiveForm.Refresh() where frmName is your form's name.
Hope it helps.

This happens not when i'm running my form , but when i'm in desigmode. Even when i run it stays the same color like before.

Its not inheriting the forecolour of the container is it?

If the forms forecolour is red, try changing the forecolour of the form and see what happens.

This will not be a normal behaviour..