Strength = Saturation + Brightness
When you say "strength" I assume you mean saturation or the purity of the hue. On a mathematical level, you can simply match the brightness and saturation values in HSB color mode.
Not so fast
But equal luminosity doesn't necessarily result in equal strength or dominance. Color theorists Johannes Itten and Josef Albers both prescribed lessons for their students that dealt specifically with this problem as a way to train one's eye for color.
You can see in this color wheel (Itten's rendition), that all the colors are presented at full saturation (allowing for the abilities of digital display). Despite their equality in this respect some are stronger than others. Yellow and red clearly make their presence known, while blue seems subtle in comparison.
Bringing balance to the force
This page from Itten's book balances the strength of colors (focusing on compliments) using scale. Notice that the warm range holds a lot of power, but green brings some punch to the party too.
The painter JMW Turner is a highly regarded for his use of color strength to create tension and drama in his work.
Piet Mondrian, on the other hand, worked to flatten color and balance the impact of various hues.
Related color issues
I've commented elsewhere about
The way colors can change one another, making three colors appear as four.
Albert Munsell's concept of carefully wielding strong colors.