If you want to create a cool multicolor gradient without much time spent, follow these steps.

# Create a high quality grayscale image.
# Use the gradient tool from darker to lighter (or vice versa) — either linear or radial.
# After applying the gradient, change color mode to Indexed color (select it from the Mode menu) — but not less than 8-bit.
# Then select any color table you want by choosing ColorTable option in the Mode menu. Good results can be achieved by choosing always-present-in-Photoshop Black Body (instant German flag gradient) or Spectrum color (instant lame effect) tables.

Image Image

If you need another gradations, make color table by yourself.
After doing all this convert your image to RGB or CMYK and continue working on it.