The model of pc doesn't really matter, any current model should be ok for a 2D painting. I bought a pc of ~$1500 USD 2yrs ago for myself to learn 3D, FYI. I scaled the canvas to 8k after the drafting stage.
How are you applying color? I have tried doing black and white and then add color but Im not liking my results and wonder if I’m using the wrong kinda blending mode or something.
I separate the elements into layers and use adjustment layers for each of them. Color Balance layer or Hue/Saturation layer (with colorize checked) will do. Draw on overlay/multiply/color dodge..etc. layers for adding more color changes. There is no easy solution that can do the trick everytime with one click, it is more important to get some color references to narrow down your goal, experimenting the adjustments with mix and matches, tweeks and twerks, and be patient :)
