I have some normal maps that are white/gray in color instead of the normal blue/purple color. I know this isn't completely Blender related since I'll probably use something like Photoshop to fix the textures, but how can I fix the color to make it the right color?
If you don't have to convert it:
You could just pop it into a Bump-Node and connect the Normal output to your BSDF. Try playing with the values, sometimes the result gets better if you additionally plug the Normal Output of the Bump-Node into a Normal Map Node. (For me) Too high Values in the Bump- or Normal Map-Node will make your Roughness look shinier than it should be so be careful with that.
If you have to convert it:
Use the same setup as above. Instead of a cube, use a plane and apply the texture with no stretching or just activate the "Import Images as Planes" addon and import the image. Your UV bounds should match the image. Go to Render Properties, make sure you are in Cycles Render, scroll down to Bake and open the menu. Once you opened it, (if activated) deactivate Selected to Active. Now add an image texture in your material but do not connect it to your material. Call it the name of the normal map, make it the needed resolution and change Color Space to Non-Color (only Base Color-textures should be sRGB). Now select that texture and hit Bake. This may take some time, depending on your hardware.
Your Image Texture should now contain the normal map. Go to Image Editor, find the Texture and save it locally as it gets lost if you don't. Now go back to Shader Editor, plug your new texture into the color socket of a Normal Map Node and plug it into the Normal input of the BSDF to see if it worked.
I hope that helps you archieve your goal!
PS: I think "Convert a Bump Map / BW-Texture into a Normal Map inside Blender" might be a more accurate title, if i understood your question correctly