I'm trying to make a document with one color black (c:0,m:0,y:0,k:100) background with red text on it. However in CMYK the text has this weird grey white-ish edge. In RGB it doesn't show but in CMYK it shows. I'm wondering will this show when printing or do I need to use different methods? ie. using rich black etc.
Here's the RGB and CMYK comparison
This is because in CMYK images where you have two distinct colours, a gap can appear when the image is printed. Basically, there is no black ink in your red text (which is hardly surprising!).
What you need to do is employ trapping when outputting your CMYK image, to help close up the gap. Trapping is basically an outline around the text to help mitigate this problem. When printing, this trapping will also help if there is a slight misalignment between the plates on the printing press.
It's really easy in Photoshop with a file like yours to trap text against a background.
Before outputting the finished CMYK image for print - click Image > Trap. 1 px should be enough. This will flatten all layers and rasterize everything, so make sure you don't overwrite your original image!
Examples: Untrapped (left) and trapped (right)
When designing for CMYK you need to remember the drawbacks and requirements of changing RGB to CMYK.
As you can see your RGB red have this burgundy outline. Which, when changed to CMYK change to not so full black - grey. This is probably from profile you are using.
But anyway. First thing first. Don't do that in RGB if you don't have to. This is so simple thing that it don't need RGB tools.
Second, when you are using Rich Black you need to remember that all colors will be knocked out (if not selected otherwise). So there may be some outline between text and background when plates are not aligned. So you should set "trap" that will be overprinted on the background.
It is difficult to know without seeing the real values of CMYK.
That is probably happening because you are converting the r0g0b0 black to k100% only. If this is the case there is a checkbox that activates and deactivate this option. (I do not remember where is it right now)
The black should be something like c75m68y69k90. (These values depend on the color profile used)
One way you can fix this is using a controlled rich black but this must be done in CMYK mode.
The surrounding black should be the red you are using plus k100%. It is like using the overprint option if they were vectors.
One drawback is that the black will be a warm black, not a neutral one.
