An other question about required fields en color change

I've got 4 fields.

The client must enter a Country name in Field1

If this is "NL", than Field2, Field3 and Field4 become required.

I've done this with FormCalc

if (Field1 == "NL") then
Field2.validate.nullTest="error"
Field3.validate.nullTest="error"
Field4.validate.nullTest="error"
else
Field2.validate.nullTest="disabled"
Field3.validate.nullTest="disabled"
Field4.validate.nullTest="disabled"
endif

Now comes my problem:

When field2, field3, and field4 are requered, they have to change color (fill color red) when empty

And when they are not required, they don't have to change color

Can anyone help me with this problem?