In multiple instances, I have made a mistake in pen on a colored or decorated piece of paper. White-Out would be too obvious because the page wasn't white to begin with. What can I do/use/lifehack that can solve this?
I've used three techniques in the past for this kind of thing.
Wipe it off. If the surface is relatively non-absorbant, then a wet-wipe should be able to lift the pen off. I've saved miswritten birthday cards in the past this way.
Abrasion. If you're writing on unwaxed paper or card, and have used a biro instead of a fountain pen, you can (very carefully) scrape away the top layer of paper immediately where you have written on. Use anything hard that won't leave a mark, and go slow! The ink will be removed with the top layer. Practice this technique before using on something important!
Paint. If you have access to a decent painting set, you can effectively make your own White-Out in whatever colour you need. Ideally, you need acrylics - it'll give you an finish very similar to White-Out. Water colours won't cut it.
If you use one particular colour often, consider investing in a POSCA paint marker. They come in a large range of colours, and will allow you to effortlessly cover over any mistakes as easily as White-Out.
Literal cut & paste a piece from a matching sheet of paper and align with the paper's pattern if necessary.
A gel pen that matches the paper you are working on. I don't use color paper much, but if a mistake is small I'll often opt for a white gel pen over white out. Much less obvious, and gives me more control in my experience.
I have occasionally used fingernail polish. It will go on the same way as White Out.
You can also get non-gloss (matte) finishes so that your repair doesn't show as much.
As a side bonus, if the polish color doesn't match the paper color, you can use it to make art on the page. This will make it look like you decided to do artwork in the middle of the writing areas (camouflage).