For your purposes, you can practically ignore the chlorphyll 'b' line, since there are only about 1/3 as many 'b' receptors as 'a', and because the chlorophyll 'b' is (considered to be) an "accessory" pigment that transfers the light it absorbs to chlorophyll 'a'.
And DO ignore the wavelengths that are closer to the baseline between deep blue and deep red peaks, because they are so inefficiently absorbed that you'll waste most of the power used to develop the light. Good luck, and happy algae growing.
It can depend upon what species of algae you are growing, but generally speaking, the strongest absorption peak for chlorophyll 'a' is in the deep blue, followed by deep red. (BTW, don't let the misplaced color spread in the picture throw you off, despite what it shows, it's 430-460nm blue, and 660-700nm red).