I'm no expert, but I think these things should help:
Use the image attached to this message to test yourself for aphantasia, or difficulty with visualization. Look at the red star, close your eyes, and try to visualize the same red star in your mind's eye. If you get level 5 or 6, great. If not, train your ability to visualize until you are at a 5 or 6, ideally a 6. I'm not sure, but I'd expect that a few hours of focused effort applied to the following activities will get you there:
1 - Train with a longer duration between stimuli (4-5 seconds) and use that time to practice holding the shape and color in your mind in full detail. Think of the inflated interval like training wheels. Then decrease the intervals / take the training wheels off when you can comfortably hold the 3 images and shapes for the full 5 seconds, or whatever interval you choose.
2 - Take ~15 minutes a day to look at something in your environment, close your eyes, and visualize that thing in full color and detail for as long as possible, using quick glances to correct yourself. The better you get, the more objects you can attempt to visualize. I'd expect that if you can visualize 3+ objects in full color and detail, you'll have no problem with the visualization required for n-back training.
Some n-back specific strategies:
1 - Visualize a static/unmoving row or column with cells, like a 1xn row or nx1 column in Microsoft excel, that each hold one image and color. The length of the row / column is equal to the n-back level. Each new input (color and shape) replaces the old input for the corresponding cell, so your minds eye is filled with a constantly changing set of n images and shapes for the duration of the "set."
2 - Assuming you are using the brainworkshop software, try changing your images to pentominoes. I believe they have the greatest average surface area of any of the 5 shape types. More surface area means more color per stimulus, which means the color should be easier to remember
Beyond what I typed above, I say just keep at it. The brain will adapt given enough stimulus and rest.