I have recently bought a potted rose plant and now its leaves have turned variegated. It was a nice bottle green (uniform) when I bought it first. Thank you in advance for your help!
The problems with your rose plant appear to be cultural as much as anything else - I would suggest you buy some decent potting soil if you must keep it in a pot, but bear in mind that, unless the rose is a miniature or patio variety, most hate being contained in a pot and don't do well, particularly after their first year, because they want to put down deep roots. The trouble with using soil from the garden or just from outdoors is it may have pathogens within it, and whilst plants cope with things like that actually in the ground, when you put the soil in a pot, there's nowhere for the pathogens to go, because its contained in such a small space. That alone may account for the problems you're seeing, but it may also be that the soil is poor in terms of nutrients. Roses like heavy, fertile soil conditions, growing best in rich, clay soils, and do not like to dry out completely, though drought conditions usually don't kill them, they just don't grow and flower very well. They are also a high maintenance plant, being prone to many fungal infections and insect infestations, and its usual to either use a preventative spray regime fortnightly during the growing season (with a combined fungicide insecticide such as Roseclear Ultra) or to inspect regularly and treat with either a fungicide or an insectide, depending on any problems seen. They also benefit from feeding in early spring and again about 8 weeks later with a specialist rose food to help keep them strong to resist all the pests and diseases they're very prone to get.
If you know that your rose is a reasonably small variety, and you want to continue growing it in a pot, on the assumption you can buy something decent to grow it in, take it out of its pot, brush off any soil you can without damaging the roots, rinse the roots in a bucket of water, clean the existing pot thoroughly, fill up with the new potting mix and replant the rose, watering well. Then I suggest you prune back to healthy growth, to an outward facing bud, and start a proper care regime as already mentioned. If you're wondering what potting mix you should use, I would recommend John Innes No. 2 or 3, but I've no idea what's available where you are.