Paint Tool SAI does not have a color mode layer which GIMP and Photoshop have.
So how do we recreate this in SAI.
1. Create your painting in grayscale.
2. Create a new layer. Set it to Multiply. Then start painting your base color. Lower the opacity to around 89%.
3. Duplicate your Multiply layer then set the mode of the layer to Lumi & Shade. Then lower down the opacity of this layer to 10% - 20% depending on your preference.
You will notice that you've now colored your painting close to the colors you desire while respecting the value in your grayscale layer.