It's not about the color of the mod by which you get the bump. What matters is the color so that you have an increased chance of getting a higher speed secondary. For example. If you acquire a gold mod with a speed secondary, you have four chances to increase that secondary speed. A random secondary stat increases at lvls 3,6,9,12. If you acquire a purple mod, you start out with 3 secondary stats. If speed is one of them then you have 3 chances to increase the speed bc the last lvl for the secondary stat at 12 will add the fourth secondary stat to the mod. So at levels 3,6,9 you have a chance to increase your speed stat. That's where the rng takes it's place. Blue mods you get two chances bc you have two secondary stats to begin with so at lvls 9,12 you will be adding the third and fourth stat. And so on for the green mod adding a secondary stat at lvls 6,9,12. That's why the pros on you tube try to encourage you to sell the grey mods to go for purple and gold mods. I hope that helps. I can try and find the video on YouTube that better explains this if anyone likes