Matplotlib, plot the column of a dataframe, and the lines changes color based on a condition from another column

I am not sure if this is the most elegant way since I am fairly new to Python, but this seems to accomplish what you are looking for.

First, import relevant libraries:

import pandas as pd
import matplotlib.pyplot as plt

Then, plot all the price data as a black line:

plt.plot(df['time'], df['price'], c = 'k')

Finally, loop over the combinations of consecutive rows of the price column (because this is really what you want to be color coded, not the points themselves). The code here will identify if the second element in each combination of consecutive elements was akin to an increase or decrease. If it was an increase, it will plot a green line over that segment of the black line. Conversely, if it was a decrease, it will plot a red line over that segment of the black line.

for val in range(2, len(df)+1):   
    start = val - 2
    focal_val = val -1
    if df['trend'][focal_val] == 'increasing':
        plt.plot(df['time'][start:val], df['price'][start:val], c = 'g')
    elif df['trend'][focal_val] == 'decreasing':
        plt.plot(df['time'][start:val], df['price'][start:val], c = 'r')

The final graph looks like this.