Slice on an element present in different fields

Hi Raph 
Please do the following

Create a filter table containing all colors 

Colors = 
SELECTCOLUMNS (
    FILTER (
        DISTINCT (
            UNION (
                VALUES ( Products[Color1] ),
                VALUES ( Products[Color2] ),
                VALUES ( Products[Color3] )
            )
        ),
        [Color1] <> BLANK ( ) 
    ),
    "Color", [Color1]
)

Then your measure would be

Number of Products = 
VAR SelectedColor = SELECTEDVALUE ( Colors[Color] )
RETURN
    COUNTROWS ( 
        FILTER ( 
            Products,
            Products[Color1] = SelectedColor
                || Products[Color2] = SelectedColor
                || Products[Color3] = SelectedColor
        )
    )