Colour of the Gabors

Hello, I have a question about the colour of the gabors.
Do I understand it correctly, that if I use RGB (values from -1 to +1) then the GratingStim just asribes a sine value to each pixel and multiplies chosen colour by this sine value? And then this value is changed by other parameters (like contrast and opacity)?

For example: If I set the colour of the gabor to be (1, 1, 0.5) then the colour of the pixels that are ascribed with sine value -0.5 will be (-0.5, -0.5, 0.25)? And then this value can be changed depending on my choice of contrast and opacity?

Thank you!