I've seen variations on some of the last few chakras as white instead of violet or purple, so I'm wondering what are the official/original - from Sanskrit (?) - colors?
Chakras in Buddhism generally stem from different sources than their equivalents in Hindu systems.
Not only do the colours vary, but even the number of chakras and the count of their petals.
Even within Buddhism alone there is no single answer- it depends upon the specific tradition and lineage that you are following.
Likewise the specific chakra that you would be using depends upon the stability and strength of your meditation practice.
Typically, working with any chakra is strongly discouraged until you are able to sustain a clear, stable, bright visualisation of 64,000 sesame seeds (each one uniquely clear and present) for about four hours. Until then, one should be working on traditional (shared) methods to develop your samatha.
As the sole purpose of chakra work in Buddhism is to further develop concentration, there is no purpose to it without this prior grounding in samatha.
The concentration via chakra developed is unusual in that it cuts off waking consciousness, and leads to dream, deep dream, or death levels: while being extremely dangerous for the unprepared and the untrained (think “defuse a bomb strapped to your mind” type of dangerous), being able to sustain vipassana (insight into the three natures of all phenomena: suffering, impermanent, empty) is particularly efficacious when doing so via a stable subtle consciousness: This is because the mind being used is ‘stripped bare’ of coarse conceptions, and produces an extremely stable and pervasive realisation.
Needless to say, any lack of understanding of the actual object of meditation must be thoroughly eliminated beforehand.
These practices are generally considered secret, because they can lead to irremediable damage, and because the requirements to make use of them are so tough that it doesn’t really make sense to share until you have got to a suitable level.
Despite the traditional secrecy, several well-renowned scholars have published details of these systems; however, if you think to learn of them for purposes of art or ‘colouring in’, I would suggest you look elsewhere.
If you are legitimately interested in the practice then, as said above, the first part of the path is in common with all Buddhist traditions: learn to do good, cease evil, recognise karma, cease wrong view, train in ethics, concentration and insight.
Then, continue your practise as shared with Mahayana: Accept the burden of responsibility for all sentient beings, take the Bodhisattva Vow that marks such an acceptance, and sustain the 18 root and 46 secondary vows of a Bodhisattva in accordance with Asanga and Candragomin.
Once you have turned away from the wheel of suffering, and work tirelessly and joyfully in your practise of meditation, then you should have no difficulty finding someone who is qualified to help you prepare for the completion practises where you may understand the meaning, purpose, and colours of chakras.
That preparation itself requires a continuous thread of concentration, 24 hours a day, of sustaining renunciation, bodhicitta, and insight. This is a dynamic form of concentration which greatly assists in (but does not replace) your development of Samatha. Your teacher will choose a tradition appropriate to your disposition.
Most people think this path impossible, or nearly impossible. But it is possible - the primary obstacle is, as ever, wrong view.