I hope someone can answer your question. Actually, I just meditate on them by feeling them. And the only colour I ever imagined was white. So I have no clue, sorry.
But for opening them, the colour shouldn’t matter, it did not for me at least.
Is the twin serpent meditation how one begins to meditate on the ida and pingala? I'm a bit confused there
You should ask someone else who knows what she/he is talking about.
I don't want to share harmful methods, so I keep my mouth shut. When the Ida & Pingala opened in my brain, My whole body was shaking, I felt like I was on drugs (extreme "bliss", but I did not enjoy it, as it was too intense), and it felt like I was about to have a seizure. So yeah, not going to happen that I share that.
The only thing I can tell you, when they open, don't fight it.
Meditate on them and let the energy flow through them.
You can thank me later for the advice, lol.
My own theory: In order for your Ida and Pingala to "open", your chakras have to be very strong, to supply enough "voltage".
But this is just my own theory and is probably wrong.