Does a 10 bit rec 709 for HD video and 10 bit rec 2020 for 4k video represent same colour..i read from some books that compared to 8 bit color a 10 bit color represents more color information, then how does the 10 bit color of rec 709 and rec 2020 differ from each other, or do they represent same color.
The bit depth is not really relevant when discussing whether two color spaces represent the same color. Bit depth is concerned with how accurately colors are represented within a space in a way very similar to how significant digits determine how accurate numerical data is. It's a little like asking if the range 1.00-10.00 equals the range 0.00-100.00 because both ranges express the numbers to two significant digits.
The information you need in order to answer whether the numbers in two color spaces represent the same colors are the primaries of the spaces in question. This determines how large the color space is — what colors the space are capable or representing. In this case rec 2020 and rec 709 are vastly different. rec 2020 is a much larger space with primaries that are pure spectral colors. For example, in rec 2020 pure red, represented by the RGB triple [100%, 0, 0] is the color of pure 630 nm light. rec 709 can't come close to representing that color regardless of bit depth. It's 100% red is significantly less saturated and no amount of bit depth will change that.