Here is what I did:

_screen.BackColor =16711808
oform = CREATEOBJECT('form')
oform.BackColor =8453888
oform.Show

The form and the screen are garish but if you then do:

oform.Hide
oform.LockScreen=.t.
oform.Show

still pretty garish, but it is as if the form contents were invisible. So basically what Nick said seems right on target. It gets pretty strange looking if you flip to another application and then back to fox.