Flow Restrictor Flow restrictor to be used on waste water line of RO systems. Choose size needed above. 1/4" Tubing Capillary flow restrictor is inserted into the drain line tubing, which is then inserted into the membrane housing. Color Selection The color and flow rate correlations are as follows: Black - 25 GPD Membrane Yellow - 36 GPD Membrane Green - 50 GPD Membrane Blue - 75 GPD Membrane White - 100 GPD Membrane Inline flow restrictor to replace defective flow restrictors or to be used when replacing your membrane with a higher or lower flow capacity.