In PowerPoint, by default, the colors assigned to each series in a chart are the theme colors Accent 1 through Accent 6, in that order.
In general, the best strategy for determining chart colors is to change these theme accent colors in the "base" or "template" presentation you open with Presentation(...).
It is, however, possible to assign specific colors to individual data points (bar, line, pie segment, etc.) for at least some chart types.
from pptx import Presentation
from pptx.chart.data import ChartData
from pptx.dml.color import RGBColor
from pptx.enum.chart import XL_CHART_TYPE
from pptx.util import Inches
prs = Presentation()
slide = prs.slides.add_slide(prs.slide_layouts[5])
chart_data = ChartData()
chart_data.categories = ['East', 'West', 'Midwest']
chart_data.add_series('Q1 Sales', (19.2, 21.4, 16.7))
chart_data.add_series('Q2 Sales', (22.3, 28.6, 15.2))
chart_data.add_series('Q3 Sales', (20.4, 26.3, 14.2))
chart = slide.shapes.add_chart(
XL_CHART_TYPE.COLUMN_CLUSTERED, x, y, cx, cy, chart_data
).chart
points = chart.plots[0].series[0].points
for point in points:
fill = point.format.fill
fill.solid()
fill.fore_color.rgb = RGBColor(255, 0, 0)
Column width in a column chart is set automatically to fill the available space.
Bar-type plots (including column-type, as here) have a .gap_width() method that allows the gap between bar "clusters" to be specified. There is also a .overlap() method that allows gap between individual bars in a cluster to be specified. This is really a second question, so I just provide the link here. If you want a fuller answer please post a separate question.
http://python-pptx.readthedocs.io/en/latest/api/chart.html#pptx.chart.plot.BarPlot.gap_width
