Is there a detailed description somewhere of the experience of going through the gold and blue fields and then white star of the cosmic mandala?
What you call the “cosmic mandala” is called the Spiritual Eye in our tradition.
Swami Kriyananda gives this guided visualization / meditation for entering the 3 fields (gold, blue-violet, and white star) that comprise the Spiritual Eye:
Concentrate at the point between the eyebrows. Visualize there a tunnel of golden light. Mentally enter that tunnel, and feel yourself surrounded by a glorious sense of happiness and freedom. As you move through the tunnel, feel yourself bathed by the light until all worldly thoughts disappear.
After soaring through the tunnel as long as you feel to do so, visualize before you a curtain of deep violet-blue light. Pass through that curtain into another tunnel of deep, violet-blue light. Feel the light surrounding you. Slowly, the tunnel walls disappear in blue light. Expand your consciousness into that light—into infinite freedom and bliss. Now there is no tunnel. There is only the all-encompassing blueness and bliss of infinity.
At last, visualize before you a silvery-white, five-pointed star of light. Surrender every thought, every feeling into this star of absolute, ever-existing bliss.
Mentally affirm: “I awake in Thy Light, I awake in Thy Light, I am joyful, I am free, I awake in Thy Light!”