Optimise memory usage with plt.imread() - python-3.x

Question

I am working with a lot of images (120k), each image is a component of RGB + yellow of a single image (so I have 30k unique images all break down in 4 images: one for Red, Green, Blue and Yellow)
For each image ID, I merge the 4 components (RGB + yellow) into a (M,N,4) array (where M and N are the dimensions of the image).
I work with the following code:
import pandas as pd
import numpy as np
import matplotlib.image as mpimg
import matplotlib.pyplot as plt
from os import listdir
train_labels_data = pd.read_csv('/Documents/train.csv')
def merge_rgb(img_id, colours=['red','blue','green','yellow'], path = 'train'):
"""
For each images, returns an array of shape (M,N,4)
where each dimension in the 4 are red, blue, green and yellow.
"""
merged_colour_img = []
for colour in colours:
full_path = path + '/' + img_id + '_' + colour + '.png'
colour_img=mpimg.imread(full_path)
merged_colour_img.append(colour_img)
merged_colour_img = np.dstack((merged_colour_img))
return merged_colour_img
def train_data_label(train_labels_data):
"""
From the train_labels csv file, create a list of labels, and create a large
array for the train data in same order.
"""
train_ids = [img_id for img_id in train_labels_data['Id']]
train_labels = [label for label in train_labels_data['Target']]
print ('Labels and Ids collected')
train_data = []
i=0
for img_id in train_ids:
print ('Merging Image')
train_data_img = merge_rgb (img_id)
print ('Merging done, appending the (M,N,4) array to a list')
train_data.append(train_data_img)
i += 1
print ('Done appending, going to next image')
print(i)
print('Stacking all images in one big array')
train_data = np.stack(train_data)
return train_labels, train_data
train_labels, train_data = train_data_label(train_labels_data)
# SAVE OUTPUT
data_pickle_train = pickle.dumps(train_data)
data = open("/Documents/train_data.pkl","wb")
data.write(data_pickle_train)
data.close()
data_pickle_train_labels = pickle.dumps(train_labels)
data = open("/Documents/train_data_labels.pkl","wb")
data.write(data_pickle_train_labels)
data.close()
However this code uses a lot of memory and crashes half way before all images are processed. Since I am working with images I suspect I could improve the merge_rgb function, any advice how to?
Thanks,