Bright does not have to be blue, but over the last decade the change in technology has been rapid, and bright warm-toned LED lights are relatively new. Even ten years ago it was difficult, or very expensive, to get any LED lamps that offered the same brightness as incandescent or fluorescent lamps taking up the same space at a reasonable price.
Northern have ordered a job lot of "fake tubes" with nasty blue daylight colour lighting (see the FGW HST lighting which is the absolute worst to ever disgrace any UK rail vehicle)
Since the LEDs that are used for lighting are usually based on blue LEDs, 'colder', bluer lights offer the best light

ower ratio, so early direct replacements for fluorescent tubes (such as the infamous 'fake tubes' or 'dots in tubes'), for example, tended to be blue and 'cold' just to achieve the equivalent brightness. Even now, bluer lights are often brighter for a given power input, or cheaper for a specific brightness, which is why LED streetlights are usually in the cool 'moonlight' spectrum (though I've seen warmer ones recently).
Since LEDs spread first to commercial premises such as offices and workshops, which often used very bright, cool white fluorescent lamps for task lighting anyway, bluer 'daylight' tones with a typical colour temperature of 5000 Kelvin were often seen as a benefit: most business owners are more concerned about productivity than making the place cosy. Many people are now used to this quality of light.
It's only since LEDs have moved into hotels and to the domestic realm that there has been significant development of warmer tones closer to the 2700 Kelvin colour temperature of incandescent bulbs (it's less than ten years since Philips managed to market
the first LED bulb equivalent to a 60W filament lamp).
What is suitable for a train? As long as it meets the brightness standards - which both warm and cold tones can - it depends if you want to create an industrial or domestic atmosphere, or something in between. It is noteworthy that FGW replaced their cool white lights with warm white lights in their GWR "gentlemen's club" HST first class refurbishment to create a warmer atmosphere (and made the lights dim and brighten automatically depending on the amount of daylight), and that IETs use a compromise colour temperature somewhere
between daylight and incandescent.
For what it's worth, I greatly prefer a warmer tone (with a good colour rendering range) to a cooler tone, because I like to relax on trains rather than attempt to assemble electronics, paint landscapes or conduct surgery. As an aside, I worked for a company where we specified warm white lights because the 'cosier' ambience cheered people up and anecdotally reduced requests to turn on the heating in the spring and autumn!