Oil Change (red color)

Recently I bought a car and after 6000km I decided to change the engine oil and the oil filter. When I was checking the dipstick the oil level and the color were fine. However, I was surprised when I changed it because the color of the oil was red????? On the sticker was written that the oil in the engine is Castrol 20w-50 GTX (the oil that I changed). The car goes well and does not burn oil at all. I have never seen oil with that color, so I was wondering what can be the reason for that???
The new oil that I put in the engine is “Castrol 10w-40 magnetic”.