How to represent "Colors"

Have you tried stripes? If you have them at the top corners of a card it makes it easy to see in hand. By making them stripes or dashes you can really play with the quality of line as well, either for flavour or visibility (also super easy to illustrate and uses little card space). You could use them as a background image at a card filling scale and kind of blur them out, but this could have visibility issues. Another advantage of stripes is that it would be relatively easy to use multiples or order to signal different card information (this dagger has 1 red line vs this other dagger with 3 OR this dagger has red then blue vs this other dagger with blue then red)

How are the cards being used? What do the colours represent?

I like to use the borders and background. Have the dagger image against a red background?

In addition to any other ideas (and I like the backgrounds and borders idea myself) you could also label the cards, Red Dagger, Blue Dagger, etc.

How about putting the colors in each of the top left and right corners? Just a triangle colored corner like how a poker card is displayed. Meaning instead of seeing a #2 at the left corner, put a red triangle or what ever colored is being used?

How about a bulls eye of sorts? A circle of one color surrounded by however many rings are needed, and if cards have a dominant color that could be the one in the center.

Or just come up with 5 different icons, of different colors, and place them sequentially on the card, you could overlap them ever so slightly to save a bit of space and perhaps give a cleaner look. This would also be color blind friendly.