Visit color-hex and type in “rgb 144 69 149” (without quotations). Then scroll down to the “Monochromatic Colors of” section.
Real-world example: I’m designing a t-shirt graphic and found a purple camouflage pattern on Deposit Photos. But the purple is a little too much on the blue side of purple. I need to find a different main purple and create a monochrome palette.
Using Printful‘s color swatch t-shirt, I chose their purple (R144, G69, B149). Then plugged this RGB number into color-hex (as: rgb 144 69 149), which converts the RGB into HEX and lists a monochrome palette in color-hex‘s “Monochromatic Colors of” section.