I don't know how to change color of flashlight on ps4, when you are without power armor

If talking about the pip boy light. It uses the same color as the pip boy display. You can adjust in the options menu under the display tab. I quite often set it to a light blue or white (or some shade of gray).

If you have a mining helmet I believe you can adjust the color in the armor workbench. I do not recall if you need a perk for any of the colors.

So I thought that there is a way to change color of pip-boy light and color of pip-boy display independently of each other

The light is coming from the backlight of the display. You are just making the screen brighter. I prefer using a mining helmet.

I am not on the PS4 (Have the PC and Xbox 1 version). Hopefully in a DLC or in a patch we get more options to maybe mod the pipboy or adding flashlights and being able to attach them to any head gear or maybe on the chest piece.

I have a feeling mods will be made for flashlights and the good thing is that mods will be coming to the consoles as well.