To set an attribute that doesn’t change during the animation (the particle color), you’ll use a creation expression. A creation expression executes when you go to the start time. It doesn’t execute while the animation plays. (For emitted particles, a creation expression executes for a particular particle when it is emitted.)
When you use an expression to control particle attributes, make sure the selected object in the Expression Editor is a particle shape node, not the transform node of the particle object. If a particle object’s transform node is selected, move the mouse pointer to the scene view and press the down arrow key to select the particle shape node.
BubblesShape.rgbPP = <<1,0,0>>;
The expression colors all particles in the object red. The double angle brackets << and >> enclose a vector that sets the red, green, and blue components of the RGB PP attribute to 1, 0, and 0. In the RGB color scheme, this gives the object a red color.