The most important thing to take in care is that I do NOT want any stamping/printing drawing on the t-shirt. I want you to create the design playing with the form of the t-shirt, making different forms with the colors to make the t-shirt simple but very good looking.
The t-shirt is to be dressed in gyms, so take in care that it must match the fitness style. People should be able to wear it in the gym, but also outside without looking rare.
Maybe helps you to know that it is going to be tight in the arms and shoulders, and a bit loose on the body.
It must represent a fresh, cool, fitness and simple style.
The two color combinations that I have though are: Slate Gray & Azure Blue // Slate Gray & Electric Yellow. Please google those shades if you don't know exactly how they are. You are also free to present your own ideas with other color combinations, but I suggest you to use only two colors to make it simpler.
Please, remember: something simple and smooth. Play with color combinations and shapes. Just colors, the logos and the name of the brand could also be a good idea if you fit it good on the design, not other words.
In some examples you’ll see words. Disregard that and focus on the rest. To have a clearer idea, take more in care the black and yellow example.
Also take in care that some examples are very fit to the body, but the one that we want will be loose. Those are just examples to show you design ideas.