Force RGB to CMYK without color changing.

"how to perserve color"

Not possible. Your colours will change. There is literally no way to preserve colours outside the gamut of an Output device.

What you should NOT do is convert to CMYK beforehand, especially if you are using a CMYK profile that is different than your output device; Once you "throw the colours aways" you won't get them back. You should leave your images in RGB throughout your process, and let the RIP for the Indigo do the conversion for you, as it could very well have a color range that is more robust than what you are doing now, and get you a closer match.

What will definitely help you is to obtain and install the ICC Profile for the Indigo in your Adobe Suite, and set it as your CMYK applications (like InDesign) you can use it to Soft Proof Colors to see what sort of (unavoidable) shift will happen at the RIP stage. You are still not converting to CMYK at this point, you are just getting a visual indication of what to expect in a simulation of sorts.

Think about this analogy. Your RGB colour image was drawn using a box of crayons with 128 colours. A typical offset press might only be able to match a box of crayons with only 48 colours. Your Indigo might be a box of crayons with 64 colours. If you draw using the "Bright Blue" crayon out of your 128, when you go to offset press, that same colour may not be available in that box of 48, so it will use the closest blue it has, while the Indigo's 64 box might have a blue that's even a closer match. So, converting your image to CMYK beforehand is like limiting your colours to those of the 48 box, where if you left it RGB, you would have a better match by using the crayons in the 64 box,

Well, since CMYK is about inks, and inks are different in each printer - how exactly is this to be printed?

If you are sending it away for professional printing, what sort of printing, how do you choose the print service, do they give you a CMYK profile?

If you are printing it yourself, what printer (please give exact model name)?

Yes, many bright colours are impossible in the CMYK inks used for printing. The exact limits depend on _which_ CMYK you use; never convert to CMYK without cinistering why, and which CMYK profile applies.