What is the current Photoshop Target, Current image mode etc when Match Color is grayed out. When something is not support for the documents current state Photoshop will gray out menu Item. Like Lab mode or CMYK may not be supported for color match of you may have more then one layer targeted etc. Photoshop grays out things to help you know what is not currently possible.
Perhaps another approach is more precise. It takes longer to describe than to do:
1. Turn off all the layers except the one which has the value of white you want t use for reference. Use the Color Sampler tool to click on the white you will use for that reference.
2. Make a note of the RG and B readings that appear in the Info panel as Sample #1. Then remove the marker from the image.
3. Turn on one of the layers you want to adjust and use the Color sampler tool to click on the area you want to match to your reference. The Info panel will show values different from those you jotted down.
4. Go to Curves, hold down Cmd+Shift and click on the marker in the image. It will place a dot on each of the single color curves in Curves (not the composite RGB curve). To make it match.the number you jotted down, choose the Red channel, for example, (then the Green then Blue) and use the Up and Down keyboard arrows to change the second group of R G & B settings in the Info panel. When they all match, remove the marker in the image. Click OK.
5. Repeat Steps 3 and 4 for each layer in which you want a match the reference white. You now have an exact match to your reference patch appearing in all layers.