A method of video encoding or decoding used by a video encoding system and video decoding system respectively, the method comprising: receiving input data associated with a current chroma block in a current color image; and if derived-mode, DM, Intra prediction is selected for the current chroma block: determining a corresponding luma block area by locating a first upper-left pixel address of the corresponding luma block area corresponding to a second upper-left pixel address of the current chroma block, wherein the corresponding luma block area corresponds to a same area as the current chroma block in the current color image; determining one luma coding mode from a sub-block of the corresponding luma block area; deriving a current chroma Intra mode for the current chroma block; and applying Intra-prediction encoding or decoding to the current chroma block using the current chroma Intra mode derived; characterized in that said determining said luma coding mode comprises determining if the luma coding mode associated with said sub-block is none of Intra angular mode, DC mode or planar mode; and said deriving the current chroma Intra mode for the current chroma block comprises specifying a default Intra mode as the current chroma Intra mode based on the determined information that said luma coding mode is none of an Intra angular mode, DC mode or planar mode. receiving input data associated with a current chroma block in a current color image; and if derived-mode, DM, Intra prediction is selected for the current chroma block: determining a corresponding luma block area by locating a first upper-left pixel address of the corresponding luma block area corresponding to a second upper-left pixel address of the current chroma block, wherein the corresponding luma block area corresponds to a same area as the current chroma block in the current color image; determining one luma coding mode from a sub-block of the corresponding luma block area; deriving a current chroma Intra mode for the current chroma block; and applying Intra-prediction encoding or decoding to the current chroma block using the current chroma Intra mode derived; characterized in that said determining said luma coding mode comprises determining if the luma coding mode associated with said sub-block is none of Intra angular mode, DC mode or planar mode; and said deriving the current chroma Intra mode for the current chroma block comprises specifying a default Intra mode as the current chroma Intra mode based on the determined information that said luma coding mode is none of an Intra angular mode, DC mode or planar mode. determining a corresponding luma block area by locating a first upper-left pixel address of the corresponding luma block area corresponding to a second upper-left pixel address of the current chroma block, wherein the corresponding luma block area corresponds to a same area as the current chroma block in the current color image; determining one luma coding mode from a sub-block of the corresponding luma block area; deriving a current chroma Intra mode for the current chroma block; and applying Intra-prediction encoding or decoding to the current chroma block using the current chroma Intra mode derived; characterized in that said determining said luma coding mode comprises determining if the luma coding mode associated with said sub-block is none of Intra angular mode, DC mode or planar mode; and said deriving the current chroma Intra mode for the current chroma block comprises specifying a default Intra mode as the current chroma Intra mode based on the determined information that said luma coding mode is none of an Intra angular mode, DC mode or planar mode.