It's a great first go at digital painting. Most dp virgins usually airbrush or use a soft brush on the mofo and end up with something fuzzy and blurry. So good job at using harder edged brushes to define the shape of the grapes.

I would agree, choosing colours are one of the hardest parts of painting. It's sort of instinctive to choose purple for grapes isn't it? But if you look at a picture of grapes you see shades of blues, bits of pink, browns etc. Also the colours can change depending on the lighting. Best to use a reference, makes it much easier for colours.