Hi Liu,
you want to highlight your editable fields means better apply the color for that particular field by using the below code while filling the fieldcatalog.
W_CAT-EMPHASIZE = 'C700'.
ex:
COL = COL + 1.
W_CAT-FIELDNAME = 'VBELN'.
W_CAT-COLTEXT = 'Invoice No.'.
W_CAT-COL_POS = COL.
W_CAT-EMPHASIZE = 'C700'.
APPEND W_CAT TO PT_FIELDCAT.
CLEAR W_CAT.
Regards,
Krishna