I purchased a rose plant from nearby nursery for pretty cheap with big pot. It had one flower at that time, After 5 days of purchase 3 new flowers bloomed. They were dull and had curled peatals and a smaller size. This is how the old flower looked when I got it from nursery ( Middle one) The other 3 flowers were about to bloom
English rose is not a varietal name I'm afraid, not specific to a variety of rose. However, it seems what you are concerned about is the flower variation in terms of their colour; first, rose flowers only last a few days after they appear (up to 5 or 7 days max for each bloom); second, there is always some small variation in the colour, particularly as your rose has picotee flowers, so you might see some with quite a pronounced deep pink edge to the petals as the flower opens, or you might see some with barely detectable deep pink edges. This is quite normal, but add to that the change that takes place in a bloom as it ages (fading colours, changing structure) then it's possible to have 3 blooms on a rose which opened a day or two after one another and look different from one another. There is nothing you can do to change or fix the colour of the flowers - the ones currently present should be clipped off as their petals start to fall.
I asked about the variety of the rose to try to determine its eventual height and spread; there are few roses which do well in pots after a year or so, they prefer to be in the ground. In terms of fertiliser, assuming you are in the Northern hemisphere, now is not the time to give any; the best way to fertilise roses is to use a specialist Rose Food, which is usually a powder or granular formulation and given twice, once in Spring as new growth gets going, and again 6 weeks later.
With regard to watering, assuming there are drainage holes in the pot, it's next to impossible to over water.