Why is my laptop's screen colour is faint yellow

This might be a stretch for the "3-digit" consumerized commercial lineup, but I have seen EliteBooks with the SureView display do it as they age on bum panels. They shouldn't do it in practice and it's not a common issue on them, but if the LED array or privacy shield degrades on them, they can yellow enough if you know what pure white looks like on them you'll know something is wrong with the display. If it's SureView equipped, it's likely a bad LCD.

If it's not SureView equipped, check the laptop on a external monitor - if it happens there, the motherboard is going bad. If not, the LCD or LCD cable needs replacement. Start with the cable as it's cheaper then the LCD. If the LCD is bad the non SureView models use a standard iDP 30-pin display you can find anywhere as a FHD 1080p 45% NTSC IPS part. HP's PartSurfer pricing is high relative to the value of these machines, so see if you can find it elsewhere. The HD (768p) HP part number is M28174-001 and FHD is M28173-001