I had this exact problem with a redstar, and it may be the same as in the set of questions about the hen with the fluid-filled belly that meriruka posted.
A had a hen at the vet when she started gasping and quickly went purple. The vet told me to get her on her feet, that her abdomen was filled with fluid and that the fluid-filled abdominal cavity was pressing on her lungs. Birds, he said, have no diaphragm. In us that is a thick muscular wall that separates the abdominal and chest cavities.
I supported the hen on her feet, holding her under her ribs so that I did not press on her belly. Her breathing returned to normal, her color returned to normal. She actually lived a good year or so after that and then the same condition got her again.
Check the belly. If it is distended, get the pressure off her belly. If she is lying on it that might be the problem. Check the thread on the hen with the fluid in her belly if it seems that might be your problem.