This is probably the biggest barrier I've seen in looking at the work of many fluid painters. I tend to generally stay with a very similar color palette because I found what works for me. And with fluid painting, the results are unpredictable and you often get something totally unexpected using the same paint. Usually I have about 5-7 colors that I tend to use regularly, so I keep them mixed up and ready to go at all times.
Here are some of the problems I see with other artists. The work gets muddy. Usually this is caused because the artist is using too many colors that have close values. What this does is cause very little contrast between colors, so you can't see any definition with your cells or shapes. Often the overall painting will look "gray" if it has too many similar colors.
The colors used aren't sophisticated. If an artist is grabbing a lot of primary colors, those colors often don't have interesting nuances. If you think about paint swatches at Home Depot, you can understand there is a huge amount of variety out there with colors. They don't need to just be what you find in a box of Crayolas.
Go easy when mixing two different colors to achieve a third color. Or avoid it altogether. Keep it PURE. I recommend buying the color of paint you want to see in the final art, not buying paint to mix for those results. For example, it's better to buy the exact shade of green you want to use. Instead of buying blue and yellow to mix for green. It can dull your colors and reduce the contrast of the pigments. Since you want your cells to stand out (if that's what you're trying to do), contrast is important. Or you won't be able to clearly see your cells, or your work looks gray or muddy. This isn't true for all types of artwork, but it's something I practice when doing fluid painting.
What colors have you used that have produced unexpectedly good results? I've accidentally gotten great results from experimenting with certain colors, so I just keep using those same colors. I often don't know how great a color will look until the painting is covered with resin. The resin will saturate and enhance your colors.
I add a neon color to most of my paintings. The magenta or pink colors you see in my work are "neon." This goes back to the issue with contrast. A neon color will cut through the visual clutter and make the darks look darker, and the brights look crisp. I recommend a neon color but in small doses. Also, the neon color should work well with your color scheme. A lime green could be frightening unless you're grounding it in a smart color scheme.
I go easy with darker colors. In fact, I don't use black in my art (except to paint the sides). I prefer a navy, purple or dark blue. I've seen people use black really well and effectively, but I think it can easily turn a painting into mud. It can also make your color scheme look "harsh."
Purple can easily look gray unless it's grounded by other colors that bring out those reds and blues in the pigment. People ask me how I get such good purples, and it's not the brand of paint I'm using. It involves using the paint in its pure, unmixed form. And contrasting it with other colors that bring out the purple tones.
The final issue I see with some fluid artists is that they have a really basic color scheme and it never varies. So each painting tends to look too much alike. There is such a thing as being too safe. For example, I see a lot of fluid artists being stuck on a blue and white color scheme. I keep thinking, if they just inserted one more good color, it could completely change the dynamic of their work.
Getting the color right is the difference from being a good artist and a great artist.